Add Theme PAGES_ON_MENU Option

Relies on feature added by https://github.com/nairobilug/pelican-alchemy/pull/7

Before:
![before](https://cloud.githubusercontent.com/assets/3677497/3349230/69df877e-f959-11e3-8480-98e5544c74ec.jpg)

After:
![after](https://cloud.githubusercontent.com/assets/3677497/3349232/6dacf72e-f959-11e3-91c1-b3eb33a36fe5.jpg)