Change date format to ISO 8601

Currently coded as a constant in the config module. Haven't quite
puzzled out the settings system, given its complexity and reliance on
sqlite; adding a new option onto settings is really where this should
properly go. Something like this:

    geeknote settings --date_format %Y-%m-%d
    geeknote settings --date_time_format %Y-%m-%d %H:%M:%S