Provide text match metadata on search

TextMatch metadata (#109) will now appear in search results, when `SearchOptions.TextMatch` is set to `true`.

```go
options := &github.SearchOptions{TextMatch: true}
codeSearchResult, _, err := client.Search.Code("github user:google", options)

for _, codeResult := range codeSearchResult.CodeResults {
  for _, textMatch := range codeResult.TextMatches {
    fmt.Println(textMatch)
  }
}
```

* [X] Add option for `TextMatch` in `SearchOptions`
* [X] Optionally set `search` to use `Accept` header `application/vnd.github.v3.text-match+json`
* [X] Create `TextMatch` and `Match` structures for [text match metadata](https://developer.github.com/v3/search/#text-match-metadata)
* [X] Provide `TextMatches` to `CodeResult`
* [X] Provide `TextMatches` to search results for Users, Repositories, and Issues
* [X] Stringify TextMatches
* [X] Tests!