Not working under subpath

[deyigifts]

Thanks for your effort on the beautiful static image toolkit. Found that the distribution work could not work under subpath, reported file not found with absolute resource for index.js and index.css.
More detail as below.

[deyigifts]

Please help review the PR and consider merge with the fix.

[dinoosauro]

Hi, thank you for contributing on this project! I'll merge the commit now. I also see that there's an issue in the service worker, I'll try fixing it now