Add ability to whitelist domains via .configure()

Related to [#25](https://github.com/rollbar/rollbar.js/issues/25).

I had to add a few large chunks of code to util.js that I found in [release/rollbar-1.0.0-rc.10.js](https://github.com/cfurrow/rollbar.js/commit/12ca38894bbc0df25e4373817d645b11996edf3a) @ 12ca3889. Wasn't sure how to interpret the code gone missing, but still referenced in the rest of the code. Can anyone shed some light on this?

Otherwise, code is open to scrutiny. Thanks!
Looks like you may want to invert this logic. If there are multiple white-listed domains this will return `false` if the filename doesn't match the first one.

e.g. we white-list "example.com" and "example.net" in that order. The first frame has a filename of "example.net/foo". This check will return `false` before checking subsequent white-listed domains.