Add option for extra lines to include in [server:main]

[mauritsvanrees]

• I added an option clear-untrusted-proxy-headers.
• @mpeeters is adding an option max_request_body_size

Both are for for [server:main] section.

I think it would be nice to have a more general wsgi_server_main_extra option which is literally added at the end of the [server:main] section. Adding explicit options like the two above are still fine, but if we need to add our own version of each waitress option, then we have a long way to go.