Add support for non-default github branches

This allows users to specify branch names like so:

```
Flatdoc.run({ fetcher: Flatdoc.github('coinsph/api', 'README.md', 'gh-pages') })
```

which means, the default branch can stay as `master` and `gh-pages` as an orphan branch. The `ref` parameter is further described [here](https://developer.github.com/v3/repos/contents/#get-contents).