In #133 there was a request to be able to provide a custom _HookCaller (in this case in order to be able to warn on deprecated hook calls - since addressed via #138).

This feature may be quite useful in the future not only in its ability to allow users to customize the pluggy runtime but also if we want to eventually support async / coroutine based plugins (this can be done loop agnostic these days with something like multio and/or trio-asyncio).

I'd like to get feedback on this.

To start it would mean that we would make a simple change where PluginManager can take a hook_caller_type as input during instantiation.