Support range headers

[idlewan]

Range headers allow the browser to only receive some part of a file.

Useful if you want to serve videos while developing with devd, especially DASH videos: I tried to use shaka player with devd, but it needs range headers when downloading parts of the mp4 files, otherwise it throws an error.

[cortesi]

Hi. I'd pull a patch that implements this, but it's fair bit of work. There are also some subtleties to do with range headers and proxying upstream services, and our injection mechanism.