feature: press f5 to insert timestamp in user format

[rchou]

Hi, Love mg. had this feature set in emacs, based on a default feature in old windows text editor. miss it a lot in mg (osx). can we have it, please? i do see an mg command of 'display-time-mode'. just needs one builtin emacs function, 'format-time-string', and some intelligence to recognise a hotkey. mg(osx) doesn't recognise f1-f4, and thinks f5-f10 is ~. Thanks, Rahul