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/text/split.go

@@ -1,14 +1,14 @@
 package text
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "split",
 		Description: "Use suffix_length letters to form the suffix of the file name",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-a", Description: "Use suffix_length letters to form the suffix of the file name"},
 			{Name: "-b", Description: "N[K|k|M|m|G|g]"},
 			{Name: "-d", Description: "Use a numeric suffix instead of a alphabetic suffix"},