Adding an option to set the counter of a website to 0

[shirgho]

Hi Kerem,

Thank you for your extension. It has been very useful for me :)

I wanted, however, to have the option to block some websites completely as well, without downloading a separate blocking extension. I found where to make the small change (in the function decrementCounter in exploding-tabs.js) to allow this option (setting the timer check to >= 0), and can submit a pull request, if you grant me access. (or you can make the small change yourself).

Let me know if you get the time to check it, and it doesn't break the other logic.
Sameer