Support running / triggering tests for external contributors' PRs

[fbarl]

In the last couple of days, we've had two PRs (#91 and #92) submitted to this repo for which the CircleCI tests didn't run automatically.

Moreover, there was no easy way for me as a maintainer to trigger the test run so I had to copy the branch and push it to this repo to make CircleCI run the tests.

It would be best if the tests ran seamlessly also for the PRs coming from the forked repos, but a good enough start might be for the maintainers to be able to trigger them manually.

We already have this working in https://github.com/weaveworks/scope, but I'm not sure what needs to be done - maybe @bboreham knows more.

cc @palemtnrider

[bboreham]

We don't have it working in Scope.
Kubernetes is the only model I know of, where they have a separate daemon look for a maintainer writing an "ok-to-test" comment on each PR.

[fbarl]

We don't have it working in Scope.

How come I can see the tests in weaveworks/scope#3635 then and when I open them, it says an external contributor triggered them? e.g. https://circleci.com/gh/weaveworks/scope/12605?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link 🤔