Added support for search tags

Feature owners can now add extra "search tags" so that users can search by them.
Search tags are not shown to the user.

TEST=
- Add search tags such as "css-sizing,max-content" for a feature
- Search for "max-content"
- Feature shows in the results

BUG=#99

![image](https://cloud.githubusercontent.com/assets/634478/3612869/1b30f2fe-0db1-11e4-8979-a0caa0cdded0.png)