Add target directory configuration option to maven plugin

[DirkMahler]

I'm currently working on a project which shall be compiled using Java 8 and artifacts shall be made available for Java 8 and Java 7. According to the documentation the current plugin always overwrites the existing class files - it would be good to be able to specify an output directory as a configuration option.

[luontola]

This is now possible in Retrolambda 1.3.0

[DirkMahler]

Great, thanks for implementing it!