Support client-side source maps

[shadow-light]

It would be nice to be able to resolve source maps before submitting data to Rollbar. That way the source map is embedded in the application and is guaranteed to exactly match that version, and nothing needs to be submitted to Rollbar separately. I've achieved this already in this fork: https://github.com/rollbar/rollbar.js/compare/master...shadow-light:gps?expand=1

[waltjones]

Not sure whether this would be considered for the browser client side. Note that for Node, client side source maps are already supported.