List command should work for user defined maps

[johnwalker]

List currently works for builtins, but doesn't include user-defined maps. We should have a way to print them, but I'm not really sure how yet. We could have a table that specifies the origin of the nickname, like:

| origin                | dependency            | nicknames 
| builtin               | org.clojure/clojure   | clojure     
| ~/lein/plz/foobar.edn | cool-library          | cool

But we also need to think about conveying that some nicknames are being "overwritten" in the merge. So this is actually a bit hairy.