Allow filtering by categories

[ChimeraCoder]

We should allow readers to filter by categories by clicking on the category name underneath the blog post.

[rubinovitz]

Give me an endpoint and I can do the FE. This is me being an insomniac and Githubbing from my droid while in bed.

[ChimeraCoder]

@rubinovitz doesn't always have insomnia, but when she does, she counts Github issues to get to sleep. ;)

[rubinovitz]

I once read somewhere that you should never let your sysadmin boyfriend install an SSH app on his phone. You should never let a hacker girlfriend install Github on her phone.