Define SCPs supported contexts by host

At the moment when creating an SCP all requestors see the same presentation contexts. It would be nice if the supported contexts could be defined on a per-requestor basis (filtered by host/port/user/etc).

That way the services available to an SCU could be controlled by the SCP in a more user-friendly manner, allowing different levels of access.

It should be possible to use evt.EVT_CONN_OPEN with a handler to change the support contexts prior to association negotiation. Maybe add some optional plugin-like handlers?