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/rake.go

@@ -1,14 +1,14 @@
 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:        "rake",
 		Description: "A ruby build program with capabilities similar to make",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-n", Description: "Do a dry run without executing actions"},
 			{Name: "-h", Description: "Display this help message"},
 			{Name: "-I", Description: "Include LIBDIR in the search path for required modules"},