Make it search general purpose search, instead of just TODO and FIXME

[matryer]

We use TODO and FIXME by default, but we should let people specify their own code tags too.

This means people could use it for more than just TODOs:

• showing all mentions of @mazondo in the code
• showing all places where we have tagged the code with #swarm
• showing all code blocks with http://github.com/stretchr/core/issues/1 mentioned
• showing all code that uses swarm.Listen or swarm.Trigger

[matryer]

If it's all in the URL, it means you can easily bookmark the result pages.

[mazondo]

Doesn't that just make it a clone of github search?

[mazondo]

https://github.com/stretchr/sdk-ruby/search?q=mazondo&ref=cmdform