read data from yaml files

[leobalter]

Just like #155, harmonic can fetch data from yaml files.

Replacing a config.json with a config.yaml would be nice and a good alternative for other devs.

I wish I could have available time to write a PR, but I can't promise anything.

[UltCombo]

Implementing it shouldn't be too hard, but I wonder what to do in some edge cases. What if there's a config.json and config.yml in the same directory? One takes precedence over the other? Merge both of them? Throw an error? Whatever decision is taken, it would be nice to have documented.

[leobalter]

stablish an order (json first, perhaps) and a output non-blocking warning.

[jaydson]

Yep, JSON first makes sense, as it was initially supported.