Support "Forwarded" header

[victorandree]

The "Forwarded" header is the standardized replacement for X-Forwarded-For, X-Forwarded-Proto et cetera. If koa supports the latter, it should support the former when app.proxy = true.

• X-Forwarded-For is currently used for ctx.ip. It should support (and prefer) Forwarded: for=192.168.0.1
• X-Forwarded-Proto is currently used to determine protocol (for secure cookies). It should support (and prefer)Forwarded: proto=https

The forwarded-parse package can be used to parse the header, or it could be done in-house.

I could potentially write a PR for this if there's interest.

• MDN article on Forwarded header
• RFC 7239

[jonathanong]

#281