Added the option to modify the default ratio.

[igor-krawczuk]

This should give more pleasent behaviour with ~scrolling~ zooming, as at least the ratio will stay the same

This is the first time I did anything with firefox extensions, but it seemed reasonably straightforward (i didn't actually test it though since I'm at work). I can test it later, but I'd already ask you if there is anything oppsing this PR in principle (i.e. opposing the idea of changing the default)