Have trackCollection() pass the collection to the callback

[yched]

Woould be handy if trackCollection() passed the collection to the callback.

This allows using a single redux action to displatch all "collection changed" events. The reducers can then check if the collection._name is the one that interests them.

[AdamBrodzinski]

I'd welcome a PR if there was a backwards compatible way to do this (perhaps the next param?) Also including an example in the readme would be needed.

Perhaps you could even have a trackCollections method and it would take an array of collections?