Override for a specific branch

If I'm working on a project that has a `release` branch which is supposed to be compiled with stable `rustc`, and a `master` branch which is supposed to be compiled with nightly, multirust should allow me to set version overrides depending on the current branch. Example:

```
$ multirust default stable
$ multirust override nightly
$ git checkout release
$ multirust override --branch stable
```

Now everything outside of this repo and the `release` branch of this repo uses the stable channel, and all other branches of this repo use the nightly channel.