WIP DSL: add :glob key to expand fileglobs

[rolandwalker]

This is a very rough/quick implementation, and is intended mostly for discussion,
as this is the DSL proposal in #4688 which has no associated PR.

Problems here include:

1. This was implemented alongside :target in artifact/symlinked.rb, which also
   contains installation logic for artifacts. A smarter implementation would improve
   separation of concerns by moving the logic for interpreting both :glob and :target
   keys into dsl.rb (or some class called from dsl.rb)

2. Only artifact sources such as link can be expanded. Similar functionality would
   also be useful (for example) in uninstall :files.

3. This interface seems clunky. It might be cleaner to have an expand method in
   the DSL for wrapping filenames:

     link expand('*/VLC.app')

   The expand method would actually call a constructor, because the action of expansion
   must be deferred until install-time. However, I think Expander.new('*') is needlessly
   verbose for Cask authors, and not necessarily more self-documenting.

   • The expand interface might also help address problem 2 above.

References: Homebrew/homebrew-cask-versions#293 (comment)

[jawshooah]

Closing for lack of interest, and because the implementation is vastly out of date at this point.