use darker background for badge
misc button appearance & privacy improvements

Adds abilities to disable showing block count, disable flashing
of button when blocking occurs, and remembering zero blocked
hosts, for privacy. Also changed badge background color to gray.

https://github.com/cwilper/http-nowhere/issues/3
https://github.com/cwilper/http-nowhere/issues/4
https://github.com/cwilper/http-nowhere/issues/5
https://github.com/cwilper/http-nowhere/issues/6