Add support for completing-read (also helm, etc)

[walseb]

Right now this package only works with ivy. There are two reasons why. One is that completing read always sorts things which makes it really slow when scanning huge directories. There are ways to fix this but I haven't bothered yet. Another small problem is that using seq-filter to filter out irrelevant entries (those who are above the current directory) instead of the :predicate option in ivy is slightly slower (I remembered measuring a 0.1 ms difference on a pretty large folder). I would rather have better compatibility than the last point though but maybe it would be good to use the ivy method instead of completing read if ivy mode is on because of it

[walseb]

Fixed!