Allow plugin execution and validation to be skipped

[phillipuniverse]

This also makes some formatting changes and moves the package for the plugin execution. Added help text to all of the existing parameters and new ones for better discovery

Fixes #11

2 new parameters:

• skipRepositoryCheck (with user property githook.plugin. skipRepositoryCheck)
  • Fixes the case of #11 where you want to probably install the hook if the .git repository is there, but don't explode if it's not there
• skip (with user property githook.plugin.skip)
  • Skips the entire plugin execution altogether