Consider supporting external dependencies

[petrbroz]

It seems that the filestoload assets in config.json can only be local (relative to the contents subfolder). It might be a good idea to support external dependencies as well, so that, for example, instead of including the same xlsx.min.min.js into every single extension that needs to generate an Excel file, we could just reference it from a CDN, for example: https://cdnjs.cloudflare.com/ajax/libs/xlsx/0.15.2/xlsx.core.min.js.

[libvarun]

we're leaving dependency management to user, i believe we should write that clearly as a note.
in each extension config external dependencies may be duplicated, because that information is just to mention this extension depends on these external dependencies.