Support unified highlighting

[chrinor2002]

Issue

Unable to set highlighting to unified. For example:

const esb = require('elastic-builder'); // the builder

const highlight = esb.highlight();
highlight.fields(['message.analysisone', 'message.analysistwo'], 'message');
highlight.type('unified', 'message'); // <--- wont wont with typescript types, nor will it work at run time if type checks are ignored.

const requestBody = esb.requestBodySearch()
  .highlight()
  .query(esb.matchQuery('message', 'this is a test'));

Proposed Solution

Add 'unified' as a string constant supported in the highlight.type() function.

As a proof of concept we tried patching this in a local development env, and it seemed to work as expected. see #206 for details.