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/git/github.go

@@ -1,19 +1,19 @@
 package git
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "github",
 		Description: "Open a git repository in GitHub Desktop",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "clone", Description: "Clone a repository"},
 			{Name: "open", Description: "Open a git repository in GitHub Desktop"},
 			{Name: "help", Description: "Show the help page for a command"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--help", Description: "Show the help page for a command"},
 			{Name: "--branch", Description: "The branch to checkout after cloning"},
 		},