Add multiple metrics functions

As discussed here: https://github.com/praekelt/go-jsbox-metrics-helper/pull/5#issuecomment-56868126
Agreed. We could do something like: ``if (Object.keys(start_trigger).length == 1) { ...``. That also seems a bit clunky. I like the look of the trigger definitions though, so I'm reluctant to change those just to make a couple of lines of code easier to write.