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/broot.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:        "broot",
 		Description: "Show the last modified date of files and directories",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--dates", Description: "Show the last modified date of files and directories"},
 			{Name: "--no-dates", Description: "Don't show the last modified date"},
 			{Name: "--only-folders", Description: "Only show folders"},