Exclude specific users from triggering 

I have never worked on a Jenkins plugin before so please excuse my approach.
Basically my change allows a comma-separated list of usernames to be configured which checked the pusher name from the GitHub post and prevents the trigger from firing if the user is in the list.

The use case for us is that we commit back to Github as part of a pipeline which is started by post commit hook. We can there use the exclusion list, configured with the name of the Github account that our CI server uses, to prevent the build from looping endlessly.