Add git recursive-grep and git rgrep aliases

Summary:
When I'm doing refactoring in webapp, I want to be able to grep through webapp
and all of it's submodules (and their submodules, etc.) in order to try to help
me figure out if it's safe to delete something.

Test Plan:
In webapp, run the following, and ensure that that give results in webapp and at least one of its submodules:

```
git rgrep "Khan Academy"
git rgrep "Scratchpad"
git rgrep "Scratchpad" -- '*.js'
```

Reviewers: alpert, csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D13904