add skip options

Hi,

we are using this plugin in an environment where we have a common base pom but sometimes builds are executed from non-git backed folders. Currently, the plugin (0.4.1) fails for us in that case. This patch adds a flag 'skipNoGit' which allows builds to succeed (obviously without setting the appropriate variables but that is fine). 

This also adds a 'skip' flag to skip execution completely. Most maven plugins have this to allow turning plugins on and off from the command line. So mavanagaiata should have that too. :-)