Added some power and convenience to [[preferred_content]] expressions.

Before, "standard" was a special case. Now it's a first-class keyword,
so you can do things like "standard or present" to use the standard
preferred content expression, modified to also want any file that happens
to be present.

Also added a way to write your own reusable preferred content expressions,
tied to groups. To make a repository use them, set its preferred 
content to "groupwanted". Of course, "groupwanted" is also a first-class
keyword, so "not groupwanted" or something can also be done.

While I was at it, I made `vicfg` show the built-in standard preferred
content expressions, for reference. This little IDE should be pretty
self-explanatory, I hope.

So, preferred content is almost its own little programming language now.
Except I was careful to not allow recursion. ;)
