Usage share-based version choices

This adds some handy shortcuts to install versions of firefox with a minimum worldwide usage share.

There are four groups as it stands;
* >= 0.1% (10.0, 11.0, 13.0, 14.0, 18.0)
* >= 0.2% (12.0, 15.0, 17.0, 19.0)
* >= 0.3% (3.6, 20.0)
* >= 0.4% (16.0, 21.0, 22.0, 23.0)

These are all based on StatCounter’s global statistics.

The intent is that you can simply install, for example, `min_point_three`, and get a reasonable starting point for testing across browsers which a minimum number of your potential users will be using.

I’d like input on the naming of these, I’m not entirely convinced they’re right, but I’m trying to follow the naming conventions used elsewhere.