publish: Polish/refactor the publishing workflows. * Add a `--preview` option. * The `--list` option gives a nicer output, and prints warnings/errors for incorrect configuration. * Moved most of the `shell` code into a base class that's reusable. * Simplified the code to log publishing to a file. * Nicer output overall, with times.

path_mode: absolute
files:
  - ../bower_components/jquery/dist/jquery.js
  - ../bower_components/bootstrap/js/transition.js
  - ../bower_components/bootstrap/js/collapse.js
  - ../bower_components/bootstrap/js/scrollspy.js