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/python/pyenv.go

@@ -1,14 +1,14 @@
 package python
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "pyenv",
 		Description: "Pyenv",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "commands", Description: "Lists all available pyenv commands"},
 			{Name: "local", Description: "Sets a local application-specific Python version"},
 			{Name: "global", Description: "Sets the global version of Python to be used in all shells"},
@@ -18,7 +18,7 @@ func init() {
 			{Name: "rehash", Description: "Performs a deployment (default)"},
 			{Name: "whence", Description: "Lists all Python versions with the given command installed"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Output usage information"},
 			{Name: "--unset", Description: "Sets the global version of Python to be used in all shells"},
 			{Name: "-l", Description: "List all available versions"},