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/iconv.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:        "iconv",
 		Description: "Character set conversion",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--help", Description: "Show help for iconv"},
 			{Name: "--version", Description: "Output version information and exit"},
 			{Name: "-f", Description: "Specifies the encoding of the input"},