Feature/transform results

[macrozone]

This MR allows to specify a transformResult function. This will invoked for every document in the list (on the collection-cursor).

Background:

Currently, showing nested properties in the table seems to be broken in griddle. With this change, we are able to flat the entries and use the path to the property:

import flat from 'flat'; // https://www.npmjs.com/package/flat
//....

  <MeteorGriddle
        transformResult={flat}
        columns={['_id', 'emails.0.address', 'profile.firstname', 'profile.lastname']}
        publication={"users.admin.list"}
        collection={Meteor.users}
        matchingResultsCount={"users.admin.count"}
      />