Unix socket support when running studio as server

[stepankuzmin]

Mapbox Studio Classic only supports port when running as server. Now we can run Mapbox Studio Classic on unix socket:

node index-server.js --socket mapbox-studio.socket

[springmeyer]

@stepankuzmin - interesting feature, thanks for providing the pull. Before merging we need some kind of test to ensure this works and does not regress ever. Can you add a test?

[stepankuzmin]

Thanks for reply @springmeyer! Should I just test that server accepts requests using socket or something else?