Add basic API proxing to testem

@airportyh I'm aware of the discussion that happened on https://github.com/airportyh/testem/issues/218 but I really needed this functionality and I couldn't get started on the plugin system that you mentioned there (nor I have seen much development in that direction, but I admittedly don't know the project very well).

I decided to pick up @kevindente work and just add a small patch to make it general, I can keep using my fork but I thought I gathered your feedback once more in case you think there's any chance to merge something like this

Thanks!