- 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