HTTP proxy support

[joepie91]

• Read RFC(s).
• Investigate to what degree real-world clients and servers conform to these RFCs.
• Investigate relation between HTTP proxies and WebDAV.
• Investigate different authentication strategies 'in the wild'.
• Consider what kind of API to implement for configuring HTTP proxies.

[stevenvachon]

Just use tunnel.

[gajus]

Just allow to configure proxy using http.Agent, e.g.

• https://github.com/TooTallNate/node-https-proxy-agent
• https://github.com/bitinn/node-fetch

This makes it compatible with the existing HTTP clients and it detaches the implementation from bhttp core.