[VM] Default account for imap

[blueman]

Given that there is a concept of a vm-imap-default-account, I think it
might be helpful if such a default could be used to shorten references
to imap folders on the default account, especially since most people are
probably just using the default account the majority if not all of the
time.

So for example, when doing a vm-visit-imap-folder, why not allow
:<folder-name> to be shorthand for <vm-imap-default-account>:<folder-name>?

Clearly, avoiding having to type the vm-imap-default-account each time
would be a real savings. The result would be to simplify rather than
complicate referencing such folders for most people, most of the time.

The functionality could trivially be added by inserting the following
simple code snippet just after the folder name is read in:

(if (char-equal (string-to-char folder) ?:)
    (setq folder (concat vm-imap-default-account folder)))

Fast, short, and simple...

(Note I believe this should be faster than a regexp type of match)