Allow multiple reporters

[pbiggar]

I'm using elm-test on CircleCI, and it has a feature which ingests junit files, and over time produces stats about flaky tests, etc. I want to enable this. However, I also want console output, because it's simpler for devs to read in the build output.

I tried to enable both:

elm-test --report=junit --report=console

and got

The --report option must be given either 'console', 'junit', or 'json'