Feature/highlight query

This PR adds support for a few missing highlighting features:

* [highlight_query](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-highlighting.html#_highlight_query)
* [matched_fields](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-highlighting.html#matched-fields)

Also added a `HighlighterType` enum for setting the highlighter type on a field.

Closes #854