enable `license :freemium`

This is styled as a PR, but intended for discussion.

Some of our previous license discussions have been too broad; it is hard to figure out four things at once.  Let's approach one point here:  is `:freemium` useful?
* does it cover many Casks?
* can it be clearly and objectively defined?

If yes, we can document it, as it [already exists](https://github.com/caskroom/homebrew-cask/blob/2db68b6bfa7c380db4c9ac5883bafc408c561195/lib/cask/dsl/license.rb#L15).  If no, we should delete it from the code.

As a point of discussion, there is a lengthy Wikipedia page defining [Freemium](http://en.wikipedia.org/wiki/Freemium), which might be considered in favor or against.  It does mean that the term is commonly used.  On the other hand, we are not like Wikipedia.  Their [detailed breakdown of   software distribution models](http://en.wikipedia.org/wiki/Software_distribution#References) is exactly the sort of tarpit we don't want to step into.

(Incidentally `license :freemium` is a special case in that we wouldn't need to do extra bookkeeping and define it into DSL 1.1.  Because it already exists as an undocumented form, it won't break code that assumes DSL 1.0.)

Refs #6426, #7917, #7923, #5586
cc @Amorymeltzer, @vitorgalvao, @ndr-qef