Allow open popups from bookmarklets. Fix #968.

Use the `openUrlInCurrentTab` message to open bookmarklets so they are opened
via `chrome.tabs.update` instead of an injected script element. This allows
bookmarklets to open popups.

This is a pull request for issue #968. Done with help from @mrmr1993's comment on that issue. Let me know if you need something changed before this can be merged.

Thank you.