Add support for removing cache entries

[kainsavage]

Based on this MDN article it is possible to remove cached items based on a duration of time.

1. Listen on page visit
2. Capture timestamp when blacklist-hit occurs
3. When blacklist processing occurs (tab closes, etc), go back and remove cached entries from that timestamp onward

This implementation may have collateral damage (if user opens blacklisted tab, then opens a bunch of non-blacklisted tabs, for instance), but destroying cache entries is rather innocuous and will not 'harm' non-blacklisted sites.

Should be optional and the default is 'on'.