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/dos2unix.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:        "dos2unix",
 		Description: "DOS to Unix file format converter",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Show help for dos2unix"},
 			{Name: "--allow-chown", Description: "Allow file ownership change in old file mode"},
 			{Name: "-ascii", Description: "Convert only line breaks. This is the default conversion mode"},