add compiler Flags support to allow CompilationLevel.ADVANCED_OPTIMIZATIONS and CompilationLevel.SIMPLE_OPTIMIZATIONS

Hi,

A little commit to support CompilationLevel.ADVANCED_OPTIMIZATIONS and CompilationLevel.SIMPLE_OPTIMIZATIONS.

With this tweak, a  gain of 30% in size is produced with option SIMPLE_OPTIMIZATIONS set by default.

based on Java Compiler command line code.

details in documentation.

I will make a second pull request to support this feature with sbt 0.11.2 in a branch, can you update it?

thanks,

Alexandre Richonnier