refactor(commands): separate commands and its core into two folders (#32)

- I think it would be better if one for commands only, one for the spec
handling core logic so people won't be confused by the core logic inside
the commands folder (cleaner I guess)
- Regen docs

commands/js/react_native.go

@@ -1,14 +1,14 @@
 package js
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "react-native",
 		Description: "Attempt to fix all diagnosed issues",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--fix", Description: "Attempt to fix all diagnosed issues"},
 			{Name: "--contributor", Description: "Output usage information"},
 			{Name: "--entry-file", Description: "Path to the root JS file, either absolute or relative to JS root"},