Test runner invocation, ES2016, Coveralls repetition

Some thoughts on the tutorials:
- Both the mocha and AVA tutorials come with a `--timeout` option in the example. This isn't necessary for nyc support
- It seems to me the Coveralls sections could be in their own document, with links
- Especially for AVA it's likely that users want to set up ES2015 support, so we should link to that tutorial

I should have some time to make these changes if folks think they're a good idea.

(Nice site by the way!)