Allow direct specification of controller types

Since the developer is likely creating all their controllers in the same script, allow the developer to specify the list of controller types explicitly.

Taking this further, it should be possible for a developer to create singleton instances to be resolved directly, but that will have to wait for a later commit, should this first step be accepted.

To see where I'm headed, see [frank](https://github.com/frank-fs/frank/blob/webapi/src/Frank.Web.Http.Dispatcher.fs#L122).