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/jvm/mvn.go

@@ -1,14 +1,14 @@
 package jvm
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "mvn",
 		Description: "Maven - a Java based project management and comprehension tool",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--also-make", Description: "Also build projects required by project list"},
 			{Name: "--also-make-dependents", Description: "Also build projects that depend on projects in the project list"},
 			{Name: "--batch-mode", Description: "Run in non-interactive (batch)"},