Make the path of the output of optimizeJS configurable.

Exactly in the same way that paths of the outputs of
packageXyzJS are configurable, i.e., through
(moduleName in optimizeJS), (crossTarget in optimizeJS) and
(artifactPath in optimizeJS).
Merge pull request #208 from sjrd/customize-opt-js-output

Make the path of the output of optimizeJS configurable.