Add a help mojo.

This adds an additional goal to the plugin: help

mvn pl.project13.maven:git-commit-id-plugin:2.1.8-SNAPSHOT:help

displays an overview of the plugin and its function.

mvn -Ddetail=true -Dgoal=revision pl.project13.maven:git-commit-id-plugin:2.1.8-SNAPSHOT:help

displays detailed help for the revision goal.
Merge pull request #89 from hgschmie/help-mojo

Add a help mojo.