* improved debugging/logging support

- add mu4e-toggle-logging to enable/disable logging
  - add mu4e-show-log to view the log buffer, and bind it to '$' in the
    main/header/view buffers
  - add a note about it to the doc (mu4e.texi)
  - make mu4e-log the the new logging function, use colors to distinguish
    incoming/outgoing data
  - only keep last 1200 lines of log in the log buffer

emacs/mu4e-main.el

@@ -44,6 +44,7 @@
     (define-key map "f" 'smtpmail-send-queued-mail)
     (define-key map "U" 'mu4e-update-mail-show-window)
 
+    (define-key map "$" 'mu4e-show-log)
     (define-key map "H" 'mu4e-display-manual)
     map)