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/serve.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:        "serve",
 		Description: "Static file serving and directory listing",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Shows help message"},
 			{Name: "-v", Description: "Displays the current version of serve"},
 			{Name: "-l", Description: "E.g. serve --listen 1234"},