Allow ability to forward origin

If you are using a pre-strict_cors_mode version of swift and you
forward the origin then it doesn't work well for a CDN. Pre strict_mode
you could set a container access-control-allow-origin:* and it would
return the incoming Origin header instead of *. That mean the first
Origin would get cached on the edge and it would block out incoming
Origins even though the intention was to allow *.
Merge pull request #49 from dpgoetz/config_cors

Allow ability to forward origin