Fix ordering of arguments for filter function in README
Fix ordering of arguments for filter function in README

Ordering of arguments in the
[implementation](https://github.com/webrecorder/warcio/blob/master/warcio/capture_http.py#L185):
is `request`, `response` and `RequestRecorder` object, updated
documentation accordingly. Also, changed third argument name as
warc_writer was confusing.

closes #110

webrecorder/warcio