Feature request: allow multiple `--report` flags

I'd love to be able to run elm-test --report junit.xml --report console. This would allow me to program a single command that runs both in CI and on developmen.

Benefits of doing this over having separate commands for CI and local development:

• Less complexity (branching) in my test harness code. For example, the example command above could be copied as is into a package.json test hook and make npm test do something useful in CI and on development.
• Fewer differences between how tests run on CI and locally means there's a smaller chance that problems on CI proof difficult to reproduce locally.

It would mean that elm-test would need to be able to run XML output directly to a file, to avoid printing human-readable and XML output both to stdout.

Start at the elm-test CLI handling for --report and trace how report destinations are selected. Verify the behavior for repeated flags such as --report junit.xml --report console, including writing XML to the requested file while retaining console output; done means both reports are produced from one command.