Add proxying

[aeons]

Reimplementation of wking-io/elm-live#128

I'm not completely clear on whether this is the best way to add the feature, but it seems to work.

Eg. node bin/elm-serve.js --proxyPrefix api --proxyHost 'http://localhost:9000' works as expected and will proxy http://localhost:8080/api/test to http://localhost:9000/api/test.

[Dubbelll]

I just wanted to express my support for this feature.
I have the exact same use case as described in the original post: proxying a separate backend running on a different port than the frontend.

Thank you for making this wonderful tool and thank you for making this pull request!

[wking-io]

I am going to go ahead and merge this! We can let the users of this report any issues and I will reach out to you if they occur @aeons!