Fix highlighting and add support for storing and retrieving highlights

[esmarkowski]

This pull request fixes the issue with highlighting in the code and adds support for retrieving highlights.

CrashEvent.query_string("summary: propeller AND flight")
     .highlight(summary: {pre_tags: "__", post_tags: "__", fragment_size: 20})
     .first
     .highlights

All highlights are stored in .highlights

{"summary"=>[
    "While on a __flight__ originating",
    "inaugural westbound __flight__",
    "to feather the No. 2 __propeller__"
  ]
} 

Access highlights for an attribute with .highlights_for(:attribute_name)

CrashEvent.query_string("summary: propeller AND flight")
      .highlight(summary: {pre_tags: "__", post_tags: "__", fragment_size: 20})
      .first
      .highlights_for(:attribute_name)

"While on a flight originating ... inaugural westbound flight ... to feather the No. 2 propeller"