Add support for arbitrarily cascading dropdown menus

[traviskaufman]

Spec reference: https://www.google.com/design/spec/components/menus.html#menus-usage

Copied from google/material-design-lite#4508

[sconover]

@lynnmercier Does your team still have plans to implement this feature?

[sclausen]

Is there any place where discussions happen? I regularly see answers or close comments which lead to other issues with comments like "It's not on our roadmap" [Closed] for things which are either valid feature requests regarding the md spec or issues which haven't even been briefly discussed anywhere I can see.

I don't demand you working on this or that feature, but a little transparency in decision making would feel a bit less being snubbed.