Allow optional primary key mapping

[DefiDebauchery]

Collections are great, but they're even better when being able to quickly access a member of that collection instead of having to scan through it.

For example, using the README: your model has a an id field; setting up the Collection to utilize the id as a key would then allow you to access a specific user via something like users.get(2). Other Collections might opt for other data attributes as an indexed key.

Obviously this adds a little complexity because Collection becomes a dict instead of a list, but the utility seems worthwhile.