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/mypy.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:        "mypy",
 		Description: "Mypy is a static type checker for Python",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--module", Description: "Asks mypy to type check the provided module"},
 			{Name: "--package", Description: "Asks mypy to type check the provided package"},
 			{Name: "--command", Description: "Asks mypy to type check the provided string as a program"},