Percentage: Allow configuring transformation function

[giftig]

sha1sum is a good choice for distribution but it could be handy to provide a handful of options and the ability to provide your own class, too - so that you can pick a different but equally uniform distribution by picking a different hashing algorithm, for example, or just do it alphabetically by using identity if you'd like it to be more plainly divided for some reason.

[giftig]

Not as simple as it sounds as for "identity" there'd be no way of determining which proportion of all possible values a given value lies in. Different message digest algorithms are possible but not necessarily useful.