Support for YAML config?

**I'm submitting a feature request**

* **Loader/bundler:**
aurelia

**Current behavior:**
A JSON file is used as config for the aurelia-cli

* **What is the expected behavior?**
A pleasant config editing experience

* **What is the motivation / use case for changing the behavior?**
Being happy!

Currently bundle configuration goes into `aurelia.json`. It's very common one has to edit this file manually.

The problem with using JSON for such a config, with a bunch of settings that are non-obvious, is that JSON does not support any type of comments.

In many other config formats it's common to have comments explaining the different settings. Also it's at times extremely useful to be able to comment on "hacks" that had to be done for some random dependency.

JSON is a subset of YAML. So how about using YAML of this config instead, to make the editing experience a bit more pleasurable?

I guess one issue will be the tools that automatically edit this file and add dependencies when running `au install`. But from my experience, that never works properly anyway? https://github.com/aurelia/cli/issues/530

I would rather have a nice useful help text in the config file that explains where to add what. Thoughts?