Add support for yaml/.js configuration files

[bcoe]

We've had a few requests for yaml and .js configuration files:

https://github.com/bcoe/c8/issues/425 https://github.com/bcoe/c8/issues/420

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One issue with .yaml is that the available yaml parsers are massive, 10x the size of c8 itself.

.js configuration would perhaps be less unwieldy. We should look at how mocha approaches this (as I think there are potentially security pitfalls, we shouldn't just go with the naive approach).