Add new command, mark-all-dwim

**defun** `mc/mark-all-dwim (arg)`

Tries even harder to guess what you want to mark all of.

If the region is active and spans multiple lines, it will behave
as if `mc/mark-all-in-region`. Otherwise, it will delegate to
`mc/mark-all-like-this-dwim` or `mc/mark-all-like-this` (if the
prefix `ARG` is present).