changesreader /added fields & body support

[hexnickk]

Overview

This PR is addressing issue #263

In simple words, it is adding support for fields and body params for changesreader.

Testing recommendations

You can run this code against any running CouchDB database e.g. https://registry.npmjs.org/

db.changesReader
  .start({ fields })
  .on('changes', console.log);

GitHub issue number

263

Checklist

• [x] Code is written and works correctly;
• [x] Changes are covered by tests;
• [x] Documentation reflects the changes;