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/mix.go

@@ -1,20 +1,20 @@
 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:        "mix",
 		Description: "Build tool for Elixir",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "new", Description: "Creates a new Elixir project at the given path"},
 			{Name: "run", Description: "Starts the current application and runs code"},
 			{Name: "code", Description: "String containing code to execute"},
 			{Name: "file|pattern", Description: "The file|pattern to execute"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Output usage information"},
 			{Name: "--app", Description: "Name the OTP application for the project"},
 			{Name: "--modules", Description: "Name the modules in the generated code skeleton"},