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/sys/dtm.go

@@ -1,14 +1,14 @@
 package sys
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "dtm",
 		Description: "Plugin",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "apply", Description: "Create or update DevOps tools according to DevStream configuration file"},
 			{Name: "completion", Description: "Generate the autocompletion script for dtm for the specified shell"},
 			{Name: "bash", Description: "Generate autocompletion script for bash"},
@@ -30,7 +30,7 @@ func init() {
 			{Name: "version", Description: "Print the version number of DevStream"},
 			{Name: "help", Description: "Help about any command"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--config-file", Description: "Config file"},
 			{Name: "--plugin-dir", Description: "Plugins directory"},
 			{Name: "--yes", Description: "Apply directly without confirmation"},