Pass original filename

Hi guys. https://github.com/pythonicrubyist/creek/pull/6 from @nethod ended up solving our problem in terms of dealing with StringIO objects from a rails upload (thanks!), but unfortunately it no longer checks the uploaded file's extension, which was a useful thing.

This PR adds that functionality and updates the README, which hopefully others will find useful. For us, Creek is definitely a superior solution to Roo parsing large files, so I am excited to help lend a hand if issues arise in the future.