Patch to fix deprecated execCommand

[Aquafina-water-bottle]

execCommand seems to be deprecated and is no longer working on the latest Firefox release. This patch fixes it by using the newer clipboard API instead.

Notes:

• I haven't tested this out on any browser other than Firefox, but the caniuse page shows that it works the latest versions of basically all browsers outside of IE11, so I assume it works.
• With the newer API, I don't think the paste-target div is actually needed anymore, but I kept it here just in case.
• I'm leaving the version bumping (in manifest.json) up to you.
• Fixes #14

EDIT: For people who want an updated/rewritten alternative that works on Firefox, see lap-clipboard-inserter.