export command

• Add export command to create a build.clj file that mirrors the :clein/build config.
• Support {{git-count-revs}} in version file.
• Relax :pom-data to be usable alongside :license, as long as it doesn't define [:licenses].
• Note the Leiningen keys in README where there's a difference.
• Internal reorganization.
• Add noahtheduke.clein.cli as cli extension namespace. (Consider moving this to a separate library.)