DSL accept :unknown for homepage stanza

[rolandwalker]

And any other stanza where appropriate, for corner cases.

This is better than leaving a stanza missing, because

• it is self-documenting
• it is easy to grep for when trying to improve Casks
• backend code can give a more intelligent error on eg brew cask homepage

References: caskroom/homebrew-unofficial#8

[vitorgalvao]

Closing as this isn’t really something that is missed, and there’s really no point in having a cask without a home except for extreme corner cases in caskroom/unofficial, and even for the used example we were able to find a solution.