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/screen.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:        "screen",
 		Description: "Screen manager with VT100/ANSI terminal emulation",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "name", Description: "Name of the screen session"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-d", Description: "Does not start screen, but detaches the elsewhere running screen session"},
 			{Name: "-r", Description: "Reattach a session and if necessary detach it first"},
 			{Name: "-R", Description: "Reattach a session and if necessary detach or even create it first"},