Remove old comments
CLI mhc not to rescue ArgumentError

Since Thor 0.19, thor quits to emit ArgumentError
on parsing command line arguments. So, we can safely
remove rescue code, which was harmful for detecting
true error in mhc.
Add mission tag
Add X-SC-Alarm setter/getter
Add org-table style formatter
Set very-old-day is 1970

Google calendar seems not to accept articles
with dates older than 1970. This is necessary adjustment
for getting along with Google calendar.
Add iCalendar formatter