[#81288356] create Flask app as easy alternative to VirtualHost setup

Really wasn't looking forward to mucking with my machine's global config for the [CGI setup](https://github.com/18F/gsa-advantage-scrape/blob/0dc3235b4638ac85d6b4250cec5bd1150c65930e/README.md#other-usage), so figured I'd put a little time into this – turned out to be pretty easy. I didn't tear out the code/instructions for the other setup option, though I think this is much cleaner. Note the changed URI structure.