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/systemctl.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:        "systemctl",
 		Description: "Control the systemd system and service manager",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "list-units", Description: "List units currently in memory"},
 			{Name: "list-sockets", Description: "List socket units currently in memory ordered by address"},
 			{Name: "list-timers", Description: "List timer units currently in memory ordered by next elapse"},
@@ -60,7 +60,7 @@ func init() {
 			{Name: "daemon-reload", Description: "Reload systemd manager configuration"},
 			{Name: "daemon-reexec", Description: "Reexecute systemd manager"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Show this help"},
 			{Name: "--version", Description: "Show package version"},
 			{Name: "--system", Description: "Connect to system manager"},