Support X-Forwarded-Proto and X-Forwarded-Port

When behind proxies, sometimes the proxies terminates SSL
connections. When we are generating links, we want to be able to
create a sane link using the same port and schemes as the reverse
proxy wants, meaning we need to look at X-Forwarded-Proto and maybe
X-Forwarded-Port.

This commit adds both headers as extractors.
Merge pull request #235 from hamnis/hostport

Support X-Forwarded-Proto and X-Forwarded-Port