Spec out how extensions provide settings to us

[zadjii-msft]

This is one third of #100. We need to write up the spec for what the API looks like for extensions to provide uniform settings to us.

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Current plan of record:

• ICommandProvider has an ICommandSettings property on it
• ICommandSettings just has one property, IFormPage SettingsPage
  • We'll use that form page as what we render
• Then, we'll also provide a rich library to abstract out working with individual settings (so everyone doesn't need to write JSON handlers themselves)
  • This is a different third of the work for #100

[zadjii-msft]

This also merged in #141