Allow .yaml

[dmyersturnbull]

New Feature

First, thanks for building and maintaining this. I've been using this for a couple years, and it saved a lot of time.

Currently, Probot only looks for settings.yml. I'm wondering if it could also look for ``settings.yaml (.yaml`).
In my little CI/CD test repo, every other YAML file is `.yaml`.
It took me longer than I care to admit to figure out why settings weren't syncing.

Per the official docs, .yaml is preferred. (Per wayback, it has been since 2006).

Please describe the desired new functionality:

Probot Settings could use both.

• If both .yaml and .yml are present, error out.
• If one is present, use it.

I took a look at settings.js. I can submit a PR if desired.