Add target compilation strategy none

[prokod]

As evident from #140 it would be good to have an non opinionated scala compilation for cross building - not setting target/release scalac parameter

[prokod]

This feature request is hardly likely to be implemented in the foreseeing future due to the way Gradle and ScalaPlugin were designed and the amount of available api to work with for 3rd party plugin designers.

Currently only the main sourceset/s, that ScalaPlugin interacts with correctly adds scalac flags like -release to