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/pgcli.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:        "pgcli",
 		Description: "Host address of the postgres database",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Host address of the postgres database"},
 			{Name: "-p", Description: "Port number at which the postgres instance is listening"},
 			{Name: "-U", Description: "Username to connect to the postgres database"},