Add an optional middleware callback to alter data before sending to Sentry

I'm using raven-node in a project with the Express/Connect middleware, and I needed a way to alter the data that's being sent to Sentry in order to add information about the logged in user. This info changes depending on the request, so I needed access to the request and error objects in order to determine what to send along.

I thought this might be a generally useful feature for people who use the middleware and want to add request or error-specific info to their Sentry reporting, so I thought I'd submit a pull request.

I didn't see any tests for the middleware, but if you think this is a good idea, let me know if there's anything else I should add to the pull.