Add skip parameter to all goals

Currently, the only way to conditionally execute this plugin is by either putting it into a profile or by putting it into a conditionally included submodule. But profile-based activation may not be flexible enough and putting it into a conditionally included submodule is quite inconvenient.

Most Maven goals have a "skip" parameter that can be used to control them. It would be good if the Maven SVM goals would follow that approach as well.