Add a read-only option

We don't save any configuration nor items if this is set. This should be
used with care, and hence there's no short option for this.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

doc/manual.txt

@@ -234,6 +234,12 @@ the `-c` flag.
   Print events and appointments for the num number of days and exit. If no num
   is given, a range of 1 day is considered.
 
+`--read-only`::
+  Don't save configuration nor appointments/todos.
++
+WARNING: Use this this with care! If you run an interactive calcurse instance
+in read-only mode, all changes from this session will be lost without warning!
+
 `-s[date], --startday[=date]`::
   Print  events  and appointments from date and exit.  If no date is given, the
   current day is considered.