Make Clojars URL configurable

For people that run their own internal instance of clojars (e.g. an enterprise repository). This change from @mantree allows you to add a clojars URL into the lein-release config.

This change was cherry-picked from:

mantree/lein-release@93e663febf9f15ee24433567c2dd314373399878
(minus the project.clj changes)