Fixes #153 - Scalac plugins not fed to eclipse.

This fixes the issues as described:

* Xplugin/P settings can be specified multiple times
* Pull scalacOptions from "compile" configuration, most likely the ones we want for eclipse
* Eclipse cannot support specifying the continuations plugin currently, because it
automagically includes its own.

If I could get a spot-check from @hseeberger and @dotta for correctness.  My tests show it working pretty well.