Allow users to send flags to sort

This allows you to set `g:sort_motion_flags` to `ui` so that you can have case insensitive unique sorts. In theory this could also be translated to the linewise sort, that would definitely be uglier though. Not offended if you don't want to merge this in.
Can you update to read `any options` rather than `any other options` since none are passed by default. Also, can you provide a quick example, even just the `ui` you have in the PR description?