recursive maildir support

Currently, mu4s supports recursive maildirs of the form:
Maildir
|-- A
|   |-- B
|   |   |-- cur
|   |   |-- new
|   |   |-- tmp

but not of the form:
Maildir
|-- A
|   |-- cur
|   |-- new
|   |-- tmp
|   |-- B
|   |   |-- cur
|   |   |-- new
|   |   |-- tmp


The latter form (somewhat similar to a Maildir++ (according to wiki) form) 
is what occurs when Gmail nested labels are used with something like the 
following (incomplete) offlineimaprc (note the sep = /):
[Repository gmail-local]
type = Maildir
localfolders = ~/Maildir
sep = /
nametrans = lambda folder: re.sub('Spam', '[Gmail]/Spam',
                           re.sub('Drafts', '[Gmail]/Drafts',
                           re.sub('Sent', '[Gmail]/Sent Mail',
                           re.sub('Starred', '[Gmail]/Starred',
                           re.sub('Trash', '[Gmail]/Trash',
                           re.sub('Important', '[Gmail]/Important',
                           re.sub('Archive', '[Gmail]/All Mail', 
folder)))))))


I was able to get the latter recursive form to work with the attached patch, which 
modifies mu4e~get-maildirs-1.