Transpile helper files

[novemberborn]

From #577:

[we] should be supporting transpilation of helper files (files in the test directory prefixed with _, or in test/helpers).

This would be a great feature. Currently it can be pretty frustrating to write helpers in ES5 when you can write tests in ES2016!

It wouldn't be too much effort to change the Api implementation so it finds helper files and precompiles them. I think if they're then added to the precompiled option the forks will pick it up automatically.