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/do_release_upgrade.go

@@ -1,14 +1,14 @@
 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:        "do-release-upgrade",
 		Description: "Upgrade Ubuntu to latest release",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Show help message and exit"},
 			{Name: "-d", Description: "If using the latest supported release, upgrade to the development release"},
 			{Name: "-p", Description: "Try upgrading to the latest release using the upgrader from Ubuntu-proposed"},