allow setting the completions menu sorter

[justinkb]

an application using isocline might want arbitrarily complex sorting rules for its completions, say, based on Markov chains or even a neural network. or the application might wanna allow its users to pin certain common completions at the top.

in any case, if the application had already done the domain specific sorting before the completions are being passed to isocline, let's give them the option to make isocline not override that sorting. you've wisely decoupled that completions menu sorting from the prefix finding all along, so this change is perfectly safe.

kept this simple, allowing setting the sorter to NULL, but we might also provide other builtin sorters users can pick from if they don't want the case insensitive shortlex.