Copy a pattern's optional javascript file

[bmuenzenmeyer]

Broken out of #706 as this is an enhancement.

Given a pattern foo.mustache
if a foo.js exists
we should copy it to the output location of foo.mustache

• consider how this will work or conflict with react engine

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.