add a jekyllrb:post command

```
$ yo jekyllrb:post
[?] Name: Hello World
[?] Layout: post
 conflict app/_posts/2014-12-03-hello-world.md
[?] Overwrite app/_posts/2014-12-03-hello-world.md? overwrite
    force app/_posts/2014-12-03-hello-world.md
```