Feature: Allow for multiple -jvm-debug

[dwijnand]

I often like to remote debug, but having to restart sbt to add -jvm-debug x is annoying. What I'd like to do is set SBT_OPTS to include -jvm-debug 0 (to pickup a random port), but still allow for a specific port to be specified at the command line, eg -jvm-debug 5005 for IntelliJ.

Depends on #74 for the full effect to work

[fommil]

@dwijnand I'm very interested in making remote debugging easier from sbt. Do you have anything to say about https://github.com/ensime/ensime-server/issues/801 and your approach for launching things in debug mode?