Close #8: Properly quote shell arguments for mu find process

When a maildir folder contains spaces, or some other special
characters, the command line of `mu find` process for that folder gets
broken.  Fix that by quoting all arguments.

Thanks to PozDarek (darekce) for debugging and finding a fix.

* mu4e-overview.el (mu4e-overview--count): Escape arguments when
formatting `mu find` command line.

mu4e-overview.el

@@ -184,7 +184,7 @@ passed to CALLBACK will be 0."
                 "mu4e-overview-maildir-counter" nil
                 (format "%s find -f p '%s' %s | wc -l"
                         mu4e-mu-binary
-                        (format "maildir:/%s" maildir)
+                        (format "maildir:/%s" (shell-quote-argument maildir))
                         (if unread-only " and flag:unread" "")))))
 
           (set-process-filter