add support for index hinting

[sethjeffery]

Adds the hint method to plucky queries.

It's useful because:

• Hinting can speed up queries particularly in the (fortunately uncommon) case of when the internal query optimizer is picking the wrong index for a query. I've noted a few circumstances where a query was crippling our service because the planner wasn't being smart enough, and needed hinting.
• Hint is a well-documented command for MongoDB, it's neither deprecated nor hidden. It's even expected by Plucky, just not realised as a proper method yet.