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/fvm.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:        "fvm",
 		Description: "Print this usage information",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "config", Description: "Set configuration for FVM"},
 			{Name: "path", Description: "Path to the Flutter versions cache"},
 			{Name: "dart", Description: "Proxies Dart commands"},
@@ -28,7 +28,7 @@ func init() {
 			{Name: "spawn", Description: "Spawn a Flutter SDK version command"},
 			{Name: "use", Description: "Sets a Flutter SDK version to be used in a project"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Print this usage information"},
 			{Name: "--verbose", Description: "Print verbose output"},
 			{Name: "--version", Description: "Current FVM version"},