New flexible method of customising completing-read

(note -- this replaces my original request https://github.com/djcb/mu/pull/253)

Added the customisation option 'mu4e-completing-read-function' which can
be used to choose which method of input-with-completion will be used by
mu4e. This variable is set to the function which mu4e will use.

By default, mu4e will use ido (via 'ido-completing-read'), but any
compatible completing-read function can be used instead. Interesting
choices would include 'helm-comp-read' for Helm-based completion, and
'completing-read' for the built-in basic completion system.