Add support for type="module" script tags so we can use ES6 import/export

[jostermanAtPEW]

I have a commit providing this functionality on my local fork. Happy to make the pull request; your docs say to ask first.

Tell us about the issue you are experiencing

import / export syntax in javascript files listed in assets.js fails because they are loaded without the type="module" attribute

What behaviour would you expect?

the choice to separately list javascript assets that should be loaded with the type="module" attribute

How can this issue be replicated?

include an ES6 import in a javascript file listed in assets.js.

Screenshots / Screencast

Unexpected indentifier is Chrome's error on unexpected import

Your environment

Platform: Chrome 71
OS Version: iOS Sierra
Node Version: 10.9.0
NPM Version: 6.2.0
Astrum Version: 1.9.5