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/truncate.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:        "truncate",
 		Description: "Shrink or extend the size of a file to the specified size",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--no-create", Description: "Do not create any files"},
 			{Name: "--io-blocks", Description: "Treat SIZE as number of IO blocks instead of bytes"},
 			{Name: "--reference", Description: "Base size on RFILE"},