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/grep.go

@@ -1,15 +1,15 @@
 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:        "grep",
 		Description: "search text in files",
-		Generator:   core.FileGenerator(),
-		Options: []core.Option{
+		Generator:   spec.FileGenerator(),
+		Options: []spec.Option{
 			{Name: "-r", Description: "recursive"},
 			{Name: "-R", Description: "recursive + follow symlinks"},
 			{Name: "-i", Description: "ignore case"},
@@ -38,11 +38,11 @@ func init() {
 	})
 
 	// egrep and fgrep aliases
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "egrep",
 		Description: "grep with extended regex",
-		Generator:   core.FileGenerator(),
-		Options: []core.Option{
+		Generator:   spec.FileGenerator(),
+		Options: []spec.Option{
 			{Name: "-r", Description: "recursive"},
 			{Name: "-i", Description: "ignore case"},
 			{Name: "-n", Description: "show line numbers"},