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/tmux.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:        "tmux",
 		Description: "Format output",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "a", Description: "Attach to last session"},
 			{Name: "working-directory", Description: "The session's working directory"},
 			{Name: "bind", Description: "Bind a key to a command"},
@@ -60,7 +60,7 @@ func init() {
 			{Name: "lockc", Description: "Lock a client"},
 			{Name: "lock", Description: "Lock all clients attached to the server"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-F", Description: "Format output"},
 			{Name: "-f", Description: "Set client flags"},
 			{Name: "-d", Description: "Detach all others clients to the session"},