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/runner/rails.go

@@ -1,14 +1,14 @@
 package runner
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "rails",
 		Description: "Create a new rails application",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-skip-namespace", Description: "Skip namespace (affects only isolated applications)"},
 			{Name: "-r", Description: "Path to the Ruby binary of your choice"},
 			{Name: "-m", Description: "Path to some application template (can be a filesystem path or URL)"},