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/airflow.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:        "airflow",
 		Description: "Subcommand",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "celery", Description: "Celery components"},
 			{Name: "broker_api", Description: "Broker API"},
 			{Name: "flower_conf", Description: "Configuration file for flower"},
@@ -60,7 +60,7 @@ func init() {
 			{Name: "db", Description: "Database operations"},
 			{Name: "migration_wait_timeout", Description: "Timeout to wait for db to migrate"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Show this help message and exit"},
 			{Name: "-A", Description: "Broker API"},
 			{Name: "-D", Description: "Daemonize instead of running in the foreground"},