Improve registerSetting

[SachaG]

The idea behind registerSetting() is to let a package or bit of code define the settings it'll rely on, optionally providing default values and descriptions.

This would be better to do in a package.yaml file at the root of the package that centralizes all info about the package; or even maybe in the existing package.js.

[eric-burel]

You might want to check this: VulcanJS/vulcan-next#9
More broadly it would be awesome to have a standard for Vulcan packages, settings one of the field.