Feature: pass credentials to login

Web apps often have a large chunk of their functionality hidden beyond a login screen. It'd be cool if there was a way to fill in these credentials while crawling so that we can follow up on these pages. [Simple crawler supports this](https://github.com/simplecrawler/simplecrawler#faqtroubleshooting), so all that's needed is support in the API. Thanks!