Improves the preferred config files' contract

Well this ended up being a lot more fun than I bargained for.

Basically, the config files needed an opportunity to grab the existing
Lineman configs (and exposed libs) easily. At the same time, the 
require statements they contained were really awkward and—following the
new plugin feature—inconsistent. They now should just export simple 
functions that receive a lineman object instead of worry about doing
the extension themselves.

While I was at it, I started memoizing all these config-building 
functions.

Fixes #173