Allow Adding Custom Query Parameters

This will make it cross-compatible across the Couch family (IBM CouchDB, Couchbase's Sync Gateway 1.5 & below, etc).

For example, if I were to use `_changes` API of Sync Gateway 1.5 (or below) I will not be able to use the `filter` param which is the equivalent of CouchDB's `selector`.

This feature will look like something like this:

```javascript
const db = nano.db.use('mydb')
db.changesReader.start({ filter: 'sync_gateway/bychannel', channels: 'channel1,channel2' })
```

Start at the db.changesReader.start entry point and trace how options are handled for the CouchDB _changes API. Check the existing request behavior and tests, if present, then verify that arbitrary query parameters such as filter and channels reach the requested endpoint.

Written by the indexing model from the issue text.