Allow searching on multiple fields at once

... including version, which somebody was asking about the other day.

Examples:

$ lein search "id:clojure group:org.clojure version:1.6"
$ lein search "id:postgres version:7.4"
$ lein search "id:postgres version:'7.4'"

Putting up a PR for this instead of committing directly just because there aren't any search tests, so I'd like at least 1 person to have a look & think about it before we commit to supporting this feature. I *think* it's a good idea, or I wouldn't have proposed it, but confirmation or denial would be good.