Support multiple `--reporter` options on the CLI

What are you trying to do?

Enable the perf reporter, in combination with Node.js inspector hooked up to Chrome devtools, but still have the tap reporter enabled as well to increase confidence and improve visibility into what's happened upto a breakpoint.

What actually happened?

The option currently allows only one value, so if you do try to do this today, the last one will win

$ qunit --reporter tap --reporter perf

Other information

On the web we've always allowed multiple reporters. The new QUnit.config.reporters option is also naturally not limited. And on the web we actually enable two reporters by default (perf and html).

Note that you can work around this today by calling the reporter directly from a setup using the --require CLI option

qunit --require ./test/setup.js

// test/setup.js
QUnit.reporters.perf.init(QUnit);

Start at the CLI's --reporter option and compare its behavior with QUnit.config.reporters and the existing --require workaround described in the issue. Check how the tap and perf reporters are initialized together, then verify that repeating --reporter enables both reporters without the later option replacing the earlier one.