Add log-hook.

[kenrestivo]

Add a log-hook for logging or otherwise tracking tries as they are in progress.

[kenrestivo]

I just realized due to the precise way bruce handles true/false return values, could hack around this by using (abusing?) error-hook, i.e.

{:error-hook #(log/debug (.getMessage %)}

It gets a little complected if I have to actually do error-checking in the hook too, but it's manageable, so I don't really need this mod. But it'd be nice.

[joegallo]

Yeah, I've used that trick before. I rather like your solution, though, so I'll get this merged and released soonish.