Bootswatch themes

Add the Bootswatch themes by @thomaspark as "Theme Options" in Bootstrap for Vanilla.

## Included themes

![Default](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_default.png) | ![Bootstrap](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_bootstrap.png) | ![Cerulean](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_cerulean.png) | ![Cosmo](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_cosmo.png)
---|---|---|---
![Cyborg](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_cyborg.png) | ![Flatly](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_flatly.png) | ![Journal](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_journal.png) | ![Lumen](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_lumen.png)
![Readable](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_readable.png) | ![Simplex](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_simplex.png) | ![Slate](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_slate.png) | ![Spacelab](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_spacelab.png)
![Superhero](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_superhero.png) | ![United](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_united.png) | ![Yeti](https://raw.github.com/kasperisager/vanilla-bootstrap/bootswatch/design/screenshot_yeti.png)