Pass in the Changes URL (vs the DB URL)

[BigBlueHat]

There are situations where folks proxy the CouchDB API or otherwise only expose parts of it--such as the changes feed--into their own API space. Since this library only (afaict) works with the changes feed directly and doesn't access other database bits, it'd be helpful if I could pass in the URL directly to the feed itself.

@sballesteros can speak more to this use case. 😄

[jcrugzz]

@BigBlueHat Seems like we would be able to support both cases. If a changesUrl is passed in, it would supercede the normal url + '/_changes assumption. Would definitely take a PR to add this