build-formatter is not usable from outside

[Jan Rychter]

The build-formatter macro uses new-builder, but new-builder is marked as ^:private, so build-formatter is not usable from outside the clojurewerkz format.clj file.

As a result, it is rather difficult to build new formatters in applications (my specific problem was that I need monetary amounts printed without currency symbols).

[Michael Klishin]

Please submit a PR, new-builder can be made public if needed.