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/ops/xc.go

@@ -1,14 +1,14 @@
 package ops
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "xc",
 		Description: "List tasks from an xc-compatible markdown file",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-f", Description: "Print the markdown code of a task rather than running it"},
 			{Name: "-H", Description: "List task names in a short format"},
 			{Name: "-h", Description: "Print this help text"},