Allow users to pass their own Mongo instance to the mongo-store plugin

[lilsweetcaligula]

Currently it seems like the mongo-store plugin does not support passing your own Mongo instance to it. I was thinking it would be a potentially handy feature. Especially if you want to handle Mongo connection instantiation yourself.

[rjrodger]

Valid feature, but not urgent. We need to define how to do this in general for any store - a well defined option