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/spring.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:        "spring",
 		Description: "Initialize a new project using Spring Initializr",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "init", Description: "Initialize a new project using Spring Initializr"},
 			{Name: "gradle-build", Description: "Generate a Gradle build file"},
 			{Name: "gradle-project", Description: "Generate a Gradle based project archive using the Groovy DSL"},
@@ -19,7 +19,7 @@ func init() {
 			{Name: "shell", Description: "Start a nested shell"},
 			{Name: "help", Description: "Show help for other commands"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-a", Description: "Project coordinates"},
 			{Name: "-b", Description: "Spring Boot version"},
 			{Name: "--build", Description: "Build system to use"},