raven - add flag for unhandled promise rejections

[kumavis]

It would be nice to maintain the context that an error was submitted by an unhandled promise rejection. This PR intends to be one possible solution.

First time contributor to Raven-js, looking for feedback. Happy to try another approach or add tests.

[kamilogorek]

Hey, thanks for the PR. This sounds like a reasonable idea, however, in order to make it visible in Sentry's UI, we need to pass this information in the extra object.

captureException(ex, {
  extra: {
    unhandledPromiseRejection: true
  }
})

It'd also be great if you could send the same patch for raven-node here https://github.com/getsentry/raven-node/blob/master/lib/client.js#L123-L138

Thanks!

[kamilogorek]

Thanks!