Allow filters at the workflow level for pr-triggered builds

[kevinwright]

Probably easiest to achieve by adding pr as a trigger type in addition to schedule, and allowing multiple triggers in a block.

e.g.

workflows:
  version: 2.1
  my_flow:
    triggers:
      - schedule:
          cron: "0 0 * * *"
          filters:
            branches:
              only:
                - master
      - pr:
          filters:
            branches:
              only:
                - master
    jobs:
      .
      .
      .

[halfer]

It may be worth adding your feature requests to the Ideas system, which can be found in the menu on the main site.