Service worker option

[brettz9]

It'd be great to provide a service worker which can be optionally loaded (or not loaded?) upon config.

In addition to the fact of being able to work offline, it can boost performance when online as well.

(editor/svg-editor.manifest had already collected a list of the files that we'd want cached in a service worker, though it should be double-checked for any update, as well as to fit a new directory structure I'm planning on pushing soon (for modules).)