Add ability to run headless in sdb

Another idea inspired by https://github.com/ensime/scala-debugger/issues/354#issuecomment-322897620.

While `sdb` currently requires you to use it interactively, it would be nice to have the option to run _silently_ without prompting or echoing. Tied to this, `sdb` could do something like listen to a websocket or HTTP request to submit commands to evaluate and send back results.