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/fs/lsd.go

@@ -1,14 +1,14 @@
 package fs
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "lsd",
 		Description: "An ls command with a lot of pretty colors and some other stuff",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-1", Description: "Display one entry per line"},
 			{Name: "-A", Description: "Do not list implied . and"},
 			{Name: "-a", Description: "Do not ignore entries starting with"},