Exclude commits from Multibranch Pipeline?

[Mike]

Hello, I recently setup Jenkins for a Multibranch Pipeline using a GitHub organization and a Jenkinsfile.  This works well, but Jenkins is building every commit that is pushed.  I tried to have Jenkins only build the master and develop branches, but then it won't build the Pull Requests.  Ideally, here is what I'd like to do:

* Build develop and master

* Build pull requests

* Don't build pull requests that have a specific string like 'go away jenkins'

Basically, I want to copy the functionality from GitHub Pull Request Builder.  I think the only way to do this is through a custom checkout function, right?