Add tsnode.json config support

[chris-codaio]

Since mocha doesn't give us the ability to use ts-node command line options or run code as part of the registration hook, it would be wonderful if ts-node looked at a tsnode.json config file for settings.

[blakeembrey]

See #4 too. How do you imagine this would work, as a fallback or default lookup? What do you think is limiting about using tsconfig.json and having it use extends?

[chris-codaio]

I'm specifically interested in the --fast option, which AFAICT is not a tsconfig.json setting but rather something specific to ts-node.

I imagine this setting would take effect if not overridden via env var, CLI option, or register option.

[blakeembrey]

Would you prefer a separate file or using the existing tsconfig.json file? Separate is a bit trickier.

[chris-codaio]

I'd prefer re-using tsconfig.json with a ts-node specific section if that's possible. Would keep our config in one place.

[blakeembrey]

Do you mind if I close this in favour of the other issue? I'm always open to a PR 😄

[chris-codaio]

SGTM