This commit extends the RegisterCustomCmd function to also accept a reply
parser which will be invoked for replies to the custom command via
ReadResultCmd. This allows replies to custom commands to be returned a
concrete structs instead of map[string]interface{}.
ok @jcvernaleo, @jrick
This commit moves all of the structs which deal with command results and
related functions to a new file named jsonresults.go.
This is in preparation of allowing custom results to be registered.
ok @jcvernaleo
