Add `license` stanza to DSL

References: #4688.

For forward-compatibility; intentionally left undocumented.

The `license` stanza is not free-form.  There is a limited
set of valid symbols, of which the argument must be a member.
(The set will later be documented, and probably needs amendments.)

Version numbers and variants of licenses are intentionally
ommitted.  The use-case for `license` is filtering and listing
of Casks. We want to avoid becoming a detailed repository of
application descriptions: that information can be found at the
`homepage` URL.