[WIP] Provide text match metadata on search

Work in progress, fixes #109.

* [X] Set `search` to use `Accept` header `application/vnd.github.v3.text-match+json` by default
* [X] Create `TextMatch` and `Match` structures that for [text match metadata](https://developer.github.com/v3/search/#text-match-metadata)
* [X] Provide `TextMatches` to `CodeResult`
* [ ] Tests!

Less worried about, but could come later

* [ ] Provide `TextMatches` to search results for Users, Repositories, and Issues