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/sys/ns.go

@@ -1,17 +1,17 @@
 package sys
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "ns",
 		Description: "Forces rebuilding the native application",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "next", Description: "The latest development release"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--clean", Description: "Forces rebuilding the native application"},
 			{Name: "--timeout", Description: "Specifies that you want to debug the app in an emulator"},
 			{Name: "--device", Description: "Specifies a connected device/emulator to start and run the app"},