Enhanced dynamic directory

[Jasoncckit]

trafficstars

[jeffposnick]

In general, changes to the build that are not specific to the PWA template should be made against the canonical config, which in this case is https://github.com/vuejs-templates/webpack/blob/develop/template/config/index.js#L58.

Once that's reviewed and merged, we can make the equivalent change to the config used by the PWA template.

This ensures that there's a degree of consistency between the two templates.