Support objects in `indexer.addDocToIndex()`.

Updated `indexer.addDocToIndex()` to accept a batch argument that is an object rather than a string and skip `JSON.parse` if it is already an object. (And do `JSON.parse` if it is a string.)

The use cases for this are:
  - Your data source streams objects to you. Here, you can save time by not stringifying each object to pass to `si.add`.
  - You need to convert whatever you are streaming before indexing it, and thus, you need to have it in object form.

I don't know if this has broad-reaching effects on search-index, so I'll understand if this is not cool. Thanks!