Add a field to whippet.json to document specific choices

[leedxw]

Things, like plugins, can be pinned to tags or hashes, but there's no means for a future maintainer to determine why a particular ref is being used. It might be added to a commit message, but this might be difficult to surface.

currently:

plugins": [
        {"name": "bar", "ref": "v3"},
        {"name": "foo", "ref": "v6"},
]

suggested:

plugins": [
        {"name": "bar", "ref": "v3", "note": "Version 4 incompatible with foo 6"},
        {"name": "foo", "ref": "v6", "note": "Version 7 incompatible with bar 3"},
]