Add ability to replace a template

- This feature remains undocumented because it's not a good idea in most
  cases but it enables us to replace a template. This is especially
  useful when using sqlboiler as a library since testmain
  problematically loads config in it's own magical way, divorced from
  even the way sqlboiler in "normal" mode loads it. This enables
  replacement of that mechanism by replacing it's template.

boilingcore/boilingcore.go

@@ -198,7 +198,43 @@ func (s *State) initTemplates() error {
 			return err
 		}
 
-		s.TestMainTemplate, err = loadTemplate(filepath.Join(basePath, templatesTestMainDirectory), s.Config.DriverName+"_main.tpl")
+		testMain := s.Config.DriverName + "_main.tpl"
+		s.TestMainTemplate, err = loadTemplate(nil, testMain, filepath.Join(basePath, templatesTestMainDirectory, testMain))
+		if err != nil {
+			return err
+		}
+	}
+
+	return s.processReplacements()
+}
+
+// processReplacements loads any replacement templates
+func (s *State) processReplacements() error {
+	for _, replace := range s.Config.Replacements {
+		fmt.Println(replace)
+		splits := strings.Split(replace, ":")
+		if len(splits) != 2 {
+			return errors.Errorf("replace parameters must have 2 arguments, given: %s", replace)
+		}
+
+		toReplace, replaceWith := splits[0], splits[1]
+
+		var err error
+		switch filepath.Dir(toReplace) {
+		case templatesDirectory:
+			_, err = loadTemplate(s.Templates.Template, toReplace, replaceWith)
+		case templatesSingletonDirectory:
+			_, err = loadTemplate(s.SingletonTemplates.Template, toReplace, replaceWith)
+		case templatesTestDirectory:
+			_, err = loadTemplate(s.TestTemplates.Template, toReplace, replaceWith)
+		case templatesSingletonTestDirectory:
+			_, err = loadTemplate(s.SingletonTestTemplates.Template, toReplace, replaceWith)
+		case templatesTestMainDirectory:
+			s.TestMainTemplate, err = loadTemplate(nil, toReplace, replaceWith)
+		default:
+			return errors.Errorf("replace file's directory not part of any known folder: %s", toReplace)
+		}
+
 		if err != nil {
 			return err
 		}

boilingcore/config.go

@@ -10,6 +10,7 @@ type Config struct {
 	WhitelistTables  []string
 	BlacklistTables  []string
 	Tags             []string
+	Replacements     []string
 	Debug            bool
 	NoTests          bool
 	NoHooks          bool

boilingcore/templates.go

@@ -2,11 +2,13 @@ package boilingcore
 
 import (
 	"fmt"
+	"io/ioutil"
 	"path/filepath"
 	"sort"
 	"strings"
 	"text/template"
 
+	"github.com/pkg/errors"
 	"github.com/vattle/sqlboiler/bdb"
 	"github.com/vattle/sqlboiler/queries"
 	"github.com/vattle/sqlboiler/strmangle"
@@ -109,16 +111,24 @@ func loadTemplates(dir string) (*templateList, error) {
 	return &templateList{Template: tpl}, err
 }
 
-// loadTemplate loads a single template file.
-func loadTemplate(dir string, filename string) (*template.Template, error) {
-	pattern := filepath.Join(dir, filename)
-	tpl, err := template.New("").Funcs(templateFunctions).ParseFiles(pattern)
-
+// loadTemplate loads a single template, uses tpl as a base template if provided
+// and creates a new base template if not.
+func loadTemplate(tpl *template.Template, name, filename string) (*template.Template, error) {
+	b, err := ioutil.ReadFile(filename)
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrapf(err, "failed reading template file: %s", filename)
 	}
 
-	return tpl.Lookup(filename), err
+	if tpl == nil {
+		tpl = template.New(name)
+	} else {
+		tpl = tpl.New(name)
+	}
+	if tpl, err = tpl.Funcs(templateFunctions).Parse(string(b)); err != nil {
+		return nil, errors.Wrapf(err, "failed to parse template file: %s", filename)
+	}
+
+	return tpl, nil
 }
 
 // set is to stop duplication from named enums, allowing a template loop

main.go

@@ -79,6 +79,7 @@ func main() {
 	rootCmd.PersistentFlags().StringSliceP("blacklist", "b", nil, "Do not include these tables in your generated package")
 	rootCmd.PersistentFlags().StringSliceP("whitelist", "w", nil, "Only include these tables in your generated package")
 	rootCmd.PersistentFlags().StringSliceP("tag", "t", nil, "Struct tags to be included on your models in addition to json, yaml, toml")
+	rootCmd.PersistentFlags().StringSliceP("replace", "", nil, "Replace templates by directory: relpath/to_file.tpl:relpath/to_replacement.tpl")
 	rootCmd.PersistentFlags().BoolP("debug", "d", false, "Debug mode prints stack traces on error")
 	rootCmd.PersistentFlags().BoolP("no-tests", "", false, "Disable generated go test files")
 	rootCmd.PersistentFlags().BoolP("no-hooks", "", false, "Disable hooks feature for your models")
@@ -163,6 +164,14 @@ func preRun(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	cmdConfig.Replacements = viper.GetStringSlice("replace")
+	if len(cmdConfig.Replacements) == 1 && strings.ContainsRune(cmdConfig.Replacements[0], ',') {
+		cmdConfig.Replacements, err = cmd.PersistentFlags().GetStringSlice("replace")
+		if err != nil {
+			return err
+		}
+	}
+
 	if driverName == "postgres" {
 		cmdConfig.Postgres = boilingcore.PostgresConfig{
 			User:    viper.GetString("postgres.user"),