Let vue-tsc produce a warning/error for unknown/untyped elements

[AlbertBrand]

It seems unknown elements are skipped entirely when performing the typecheck. I would actually love to see a warning or error for those cases, because it often means:

• someone accidentally broke an import
• someone forgot to register the component type globally

I think the Angular template typecheck is pretty strict in those cases and it would be a good addition for strictness.

[johnsoncodehk]

This requirement still needs some discussion, because some users may not expect this behavior.

If it should implement, I may add a new option as a switch in tsconfig.json.