Customize Highlighter class

[reginafcompton]

This PR concerns https://github.com/datamade/la-metro-councilmatic/issues/321

Relevant commit in LA Metro

Important notes:

• we cannot highlight text using Solr, if that text is not "stored" (as opposed to "indexed")
• we do not store the attachment text in the LA Metro Solr index, but we do search it and need to render the results with appropriate highlights
• Haystack provides a way to do this, but the Highlighter class considers only one word at a time - hence the need for customization.

I placed this class in Councilmatic core, since I imagine we'll need if for future/other Councilmatic instances, such as NYC

@hancush - for your reference @evz - for your review