Add theme META_DESCRIPTION option

Relies on feature added by https://github.com/nairobilug/pelican-alchemy/pull/10

Should fix this: :expressionless:

![nairobilug-duckduckgo](https://cloud.githubusercontent.com/assets/3677497/3349192/2e8304e6-f957-11e3-90a9-0bd27640d1ec.jpg)

![nairobilug-google](https://cloud.githubusercontent.com/assets/3677497/3349193/33651c4c-f957-11e3-8dca-d1e679b365bd.jpg)