Add "Accepting a GitHub Pull Request" protocol

I find that having a "pr" namespace in my branches is a much easier way to handle this.

First, in `.git/config`:

```
[remote "origin"]
  url = git@github.com:thoughtbot/guides.git
  fetch = +refs/heads/*:refs/remotes/origin/*
  fetch = +refs/pull/*/head:refs/remotes/origin/pr/*
```

Which gives me a bunch of  branches under the `/pr/` namespace that are numbered for the pull requests, so this one would be `origin/pr/209`.

That can be checked out, merged, logged, etc., as it is a commitish.

It can't be pushed to to update the PR, but there's no way to do that anyhow since we don't have commit rights to forks of this repo.

I make this easier for myself by putting it into a git template: https://github.com/calebthompson/dotfiles/blob/master/git/git_template/config and https://github.com/calebthompson/dotfiles/blob/master/git/gitconfig.symlink#L50