Update README.
[chrome] double click to open unread page.
[chrome] avoid open 2 pages upon double click on empty tab.
[chrome] avoid open two unread pages on double click.

The onClicked listener maybe called concurrently. So double click check and open
unread page should both be done in the listener function.

This bug consistently triggers if double click happens when the event page is
inactive.
[safari] double click to open unread page, bump version.