Allow configuration of MONGO_QUERY_WHITELIST

[arnauorriols]

Currently the set of allowed operators is fixed in code, which makes sense to avoid unwanted surprises in new versions of MongoDB, but makes the process of including new operators rather rigid.

For example, I now wanted to experiment with the $expr operator, but I don't have any aggregation operator available. The $eq operator is missing too.

I propose to include eq in the set of operators allowed (I assume it's just a forgetfulness), and also add a configuration called MONGO_QUERY_WHITELIST, where one can add all the extra operators needed.

What do you think?

I'll push a PR later

[nicolaiarocci]

Rather stunned that $eq has been missing, and for such a long time.