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/cc/xcode_select.go

@@ -1,14 +1,14 @@
 package cc
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "xcode-select",
 		Description: "Active developer directory for Xcode tools",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Help message"},
 			{Name: "-p", Description: "Display path to active developer directory"},
 			{Name: "-s", Description: "Set path to active developer directory"},