Maven plugin needs skip option

[rubenboadana]

It would be extremely helpful to have the skip configuration option as many other maven plugins have.

The reason of this request is that we've realized the outputFile is being overwritten on each execution no matter if the classes have been changed or not, and this leads on another plugins such as maven-jar-plugin packaging again the jar file (changed files have been spotted).

[rubenboadana]

I raised a PR with the small change: #133