Enhancement: Enhance query by using column values for comparision

[florianbepunkt]

It would greatly enhance parse-server's query capabilities if it would be possible to use column values for comparison in querys.

Right now we can only compare a column against a fixed value like so

query.greaterThan('movieRating', 5);

In a lot of use cases it would help to compare it against another column value like this

query.greaterThan('movieRating', 'anotherFieldInThisRow');

[jhamet93]

Since I've been using Parse Server, I decided to give this a shot as my first pull request and start contributing.

I'm curious on the expectations of this capability particularly when certain edge cases arise. For example, imagine query.equalTo('somefield', 'someOtherField') wants to return the documents whose 'somefield' and 'someOtherField' fields are equivalent. What if 'somefield' contains the literal value 'someOtherField' and that's actually what the users intention was to query for.

This is an edge case so I wasn't sure if that should be chalked up to a design issue by the user performing the query but was just curious based on the ideas I had at adding this capability and what situations could arise for making my test cases.

Thanks!