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/spotify.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:        "spotify",
 		Description: "CLI to use Spotify from the terminal",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "play", Description: "Resume playback where Spotify last left off"},
 			{Name: "song name", Description: "The name of the song to start playing"},
 			{Name: "album", Description: "Find an album by name and play it"},
@@ -37,7 +37,7 @@ func init() {
 			{Name: "shuffle", Description: "Toggle shuffle playback mode"},
 			{Name: "repeat", Description: "Toggle repeat playback mode"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--help", Description: "Show help for spotify"},
 		},
 	})