Allow passing custom configuration to parse_headers

[gootorov]

There are several protocols that look almost identical to HTTP (in my case, it is SIP). Often the only difference is the request-line and/or the response-line. In SIP in particular, multiline headers are the norm (allow_obsolete_multiline_headers_in_responses).

Being able to parse the request/response-line by myself and then using parse_headers with a custom configuration would be very convenient.

[gootorov]

Friendly ping! :)
If you're unable to review for the time being, that's totally fine of course. Only making sure this isn't forgotten.

[gootorov]

@seanmonstar Apologies for pinging again. Do you mind having a look at this PR? This is a small, non-breaking change. Having this merged would be really great, if possible.

If there's something that needs to be changed, such as documentation, tests, etc -- please let me know.