[O] Org-specific completing-read-function

[Allen Li]

Would it be acceptable to have an Org-specific
completing-read-function?  My use case is that I would like to use a
different completing-read function like ido of ivy for Org mode,
especially for refiling, but I don't want to modify
completing-read-function globally.

This isn't entirely unprecedented; both magit and projectile implement
their own completing-read that can be set independent of Emacs's
completing-read-function.