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/ops/rbenv.go

@@ -1,14 +1,14 @@
 package ops
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "rbenv",
 		Description: "List all available rbenv commands",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "commands", Description: "List all available rbenv commands"},
 			{Name: "global", Description: "Set or show the global Ruby version"},
 			{Name: "install", Description: "Install a Ruby version using ruby-build"},
@@ -20,7 +20,7 @@ func init() {
 			{Name: "whence", Description: "List all Ruby versions that contain the given executable"},
 			{Name: "which", Description: "Display the full path to an executable"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--unset", Description: "List all available rbenv commands"},
 			{Name: "--sh", Description: "Set or show the global Ruby version"},
 			{Name: "--version", Description: "Show version of ruby-build"},