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/date.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:        "date",
 		Description: "Display or set date and time",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-d", Description: "Set the kernel's value for daylight saving time"},
 			{Name: "-f", Description: "The format with which to parse the new date value"},
 			{Name: "-j", Description: "Don't try to set the date"},