Add ability to use third party error tracking software for expressjs

[dominathan]

Added a config property that looks like so. All error handlers (sentry, rollbar, etc.) need their error handler to be injected before all other error handling software. This code sets it if supplied by the user.

	errorTracking: {
		requestHandler: Sentry.Handlers.requestHandler,
		errorHandler: Sentry.Handlers.errorHandler
	}

[Robert-W]

This looks pretty good to me. I like the idea of allowing people to be able to inject their own middleware to support more custom behaviors. @jonterrylee Let me know what you think but I am good with merging this in.

Once we do, I want to add this as another item on #119, this would be something useful to document and looks like it simply accepts any function that returns valid express middleware. We may need to think about any warnings, such as not calling next and breaking the chain of middleware (if they do this and don't return an operation outcome for an error, their implementation may no longer be conformant). I don't think we need to code anything around this, rather just mention it in our wiki.