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/create_react_native_app.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:        "create-react-native-app",
 		Description: "Creates a new React Native project",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--template", Description: "The path inside of a GitHub repo where the example lives"},
 			{Name: "--yes", Description: "Use the default options for creating a project"},
 			{Name: "--no-install", Description: "Skip installing npm packages or CocoaPods"},