Allow json files in _data dir

Per @holic's *excellent* comment over in https://github.com/github/pages-gem/pull/64#issuecomment-42631849, any valid JSON with a `.yaml` extension in the `_data` directory is valid YAML and can be read it. Tested locally, and it appears to be true.

This pull request simply expands the allowed file extensions to be read in from `yml` and `yaml` to also include `json`.

## Todo

- [x] Tests
- [x] Update docs