Allow interception of File.Open / new FileStream() - #52

Would an event here and there be the best way to convey flexibility like this?

In other projects, patterns used are f.ex. dependency injection. I do like the proposed option better than f.ex. a duplicated settings object everywhere.

My first approach was a Func<> Property in SetupHelper, but there is no Func<> in net20. My second idea was to add a RegisterFileLocator() method for FileLocator, but FileLocator is internal and it doesn't make sense to create a new FileLocator, just to override a parameter for new FileStream(). My first implementation would have been an EventFileLocator, which would have done exactly the same.

That's why I've added the FileLocator wherever it was missing (and reasonable), and add a single place to add specific handling for opening local files (that's also, why I've chosen SetupHelper, because I guess this is the place the user has already touched to register the required modules).

This event may be replaced with something more elegant, if there are additional "global" settings or overrides but I guess for now it's the most simple and elegant solution.

I also thought about adding some kind of warning, because it's a static event, which may prevent garbage collection if not unsubscribed. On the other side, you need a very special use case to subscribe to this event, and in this case you hopefully know about the internals of discutils and the downsides of static events.

Regarding Func, it can be introduced as needed.