Box borders should be more flexible

[brunchboy]

In addition to taking a simple set of keywords, as it currently does, it should also accept a map, in which the border keywords can be assigned individual attr-specs. If the value associated with a border key is simply true or the same keyword as the border (which would be the result of looking it up in a set), a default attr-spec is used (:border-unrelated or :border-related as appropriate). If the value is another keyword, then it is looked up in the attr-spec map. If it is a map, it is used as the border attributes.

[brunchboy]

Actually, the default attr-spec is always :border-unrelated. It’s up to the caller to know if a particular border is “related”.