Provide `OnTopicOptions` class for `AddTopicSupport()`

[JeremyCaney]

The UseMvc() extensions accepts an optional MvcOptions object for configuration. We may optionally be able to centralize the most common OnTopic configuration through this (example).

[JeremyCaney]

This potentially overlaps with #13, which proposes allowing the OnTopic library to be configured via OnTopic. As such, we should take care to focus on the features of OnTopic that must be determined in code (such as elements which require delegates or type references) or which need to be known upfront (such as the connection string), as opposed to basic values that can easily be assigned once the Root:Configuration section has been successfully loaded.