Add safety check for highlighting where field is undefined

[ABWalters]

Using the following search options:

const opts = {
    query: queryParam,
    fields: ['field1', 'field2'],
    include_docs: true,
    highlighting: true,
  };

If field1 matches the queryParam, but field2 does not exist in the document, then the following exception is thrown:

TypeError: Cannot read property 'replace' of undefined
    at \node_modules\pouchdb-quick-search\lib\index.js:421:23
    at Array.forEach (<anonymous>)
    at \node_modules\pouchdb-quick-search\lib\index.js:418:33
    at Array.forEach (<anonymous>)
    at \node_modules\pouchdb-quick-search\lib\index.js:411:42
    at process._tickCallback (internal/process/next_tick.js:68:7)

Some documents in the database do not have field2.

This small check will allow for search and highlighting with inconsistent DB schema.