Don't immediately begin closing tabs on install

[codyps]

Folks installing this extension may have a large number of tabs open that would with the default settings be culled immediately.

It'd be a better user experience if they had the opportunity to adjust their settings.

More generally, this could probably be done by using min(time_since_extension_installed, tab_idle_time) as the time considered in the timeout.

[mikedld]

Another option that would work for me would be to start in paused mode after installation (showing some type of explanatory notification, if that would improve UX).

[cecton]

oh god I was going to install this extension, I'm glad I checked the issues first xD