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/flutter.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:        "flutter",
 		Description: "Available emulators",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "analyze", Description: "Analyze the project's Dart code"},
 			{Name: "assemble", Description: "Assemble and build Flutter resources"},
 			{Name: "attach", Description: "Attach to a running app"},
@@ -55,7 +55,7 @@ func init() {
 			{Name: "file path", Description: "A file path for a symbolized stack trace to be written to"},
 			{Name: "test", Description: "Run Flutter unit tests for the current project"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Print this usage information"},
 			{Name: "-v", Description: "Target device id or name (prefixes allowed)"},
 			{Name: "--version", Description: "Reports the version of this tool"},