Add proxy_set_header

[lihuimail]

To make proxy more powerful,like ngnix:
proxy_set_header Custom-Header "my custom"

[mholt]

This is a good idea. Maybe it could look like this:

proxy ... {
    header Header-Name "some value"
}

Would anyone like to work on this? Should be pretty easy.

[mholt]

Actually, hold up. I barely saw #40 which does this.