Optional tab switching by mouse scroll

Hi.

During my work with pentadactyl empowered with buftabs from time to time I run
into unpleasant situation when I scroll the page down and my pointer reaches
the bottom. When this happens, buftabs switches tabs instead of scrolling the
page which is quite annoying for me.

I implemented an option which turns tab switching by mouse scroll on/off
(buftabs-mouse-scroll) and left it turned on by default.

Could you please comment on these changes and maybe accept them in your branch?