Added ':sbt-compile-on-save-command'

[Peluko]

Hello.

I've been lately playing with Scala.js. Scala.js compiles to javascript by issuing the sbt command packageJS. So I've added an option to the project definition file that lets specify the command issued to inferior sbt when saving scala files. The option is called :sbt-compile-on-save-command:

https://github.com/Peluko/ensime-src/commit/f2503262a4779de63332fa6beeceba8005194a9f

In my current setup I use scala-js-workbench, so thanks to ensime's 'compile on save', whenever I edit a scala file on a Scala.js project, ensime automatically compiles the project and scala-js-workbech refresh the debugging browser.

[rajish]

Merged. Just give me a day to forward it to the package.