Modified incremental compiler to delete old files

Also incremental compiler now produces a list of all products including those
from previous compiles, making it much easier to use with SBT tasks that
expect all products, not just the ones that were incrementally compiled, to
be returned.
Good point.

I remember bringing this up with @richdougherty originally. For the use-case where you want to increase the things that the executor returns, you do that via A. That's what is happening here:

https://github.com/sbt/sbt-js-engine/blob/master/src/main/scala/com/typesafe/sbt/jse/SbtJsTask.scala#L277