Add a flexible helper function print_apoint() and use it whenever we
print appointments to stdout. This reduces the number of copy-pasted
code and eventually allows for specifying custom format strings.
Following format specifiers are supported:
* s: Print the start time of the appointment as UNIX time stamp
* S: Print the start time of the appointment using the "hh:mm" format
* d: Print the duration of the appointment in seconds
* e: Print the end time of the appointment as UNIX time stamp
* E: Print the end time of the appointment using the "hh:mm" format
* m: Print the description of the item
* n: Print the name of the note file belonging to the item
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>