Adding in proxyPrefix option.

This options allows for a different type of proxy, one that is server based and handles the original URL as a query parameter.

My use case is that I built a simple, caching proxy for Google spreadsheets requests for use with Tabletop:
https://github.com/MinnPost/gs-proxy

I am not sure if proxyPrefix is the best option name given that there is already a proxy parameter, but didn't want to change any existing behavior.

Thanks for the great work on Tabletop.

Sidenote: It's interesting that you were working on Flatware at pretty much the same time as I was working on gs-proxy.  They both try to solve the same problem but their paradigms are a bit different.