Add new jsStats:unpkg plugin so we can determine JS size based on bundle

[Ben G]

Currently https://bitbucket.org/atlassian/panop/src/master/src/plugins/jsStats.js calculates JS bundle size based on all JS files in the top level of the package.

It would be nice to calculate this size based on the actual JS bundle, which could be fetched from unpkg.com

This would give us more accurate JS size metrics

cc @joschafeth