add pipeline syntax for running build on pull request.

Currently pipelines are run on branches, which is great however.
It would be nice to have a feature where on a non-conflicting change the merged output could be tested and verified, something like:
Use-case:
I have a branch and someone merges into master -> I want to know the state of what the result of my merge would be. So would the project maintainer.

pipelines:
  pull-request:
    'master':   
     - step:
         script:
         - tox -e unittest

then on opening or update of a pull request the pipeline would be triggered if the target branch matches the master.