Added an UnknownLicense String constructor so that we can remember
unknown license names and report them and prevent them from being
uploaded to hackage, but not fail hard with a parse error upon
encountering an unknown license. This does not add any new licesnes
or and new valid parses of existing licenses. The parsing phase
now allows an optional version like "GPL-3" however that would still
be classified as an unknown license at the moment. The point is it's
no longer a parse error but a semantic error instead.