Add calcurse-dateutil

Add a script to iterate over a certain date range, performing operations
on each of the dates and printing the result. An additional string can
be appended to each of the printed lines.

The script can be used to create "extended recurrent events". For
example

    ./calcurse-dateutil.py --date 2017-01-01 --range 365 --unique \
        --append ' [1] Test event' bom next-weekday 3 skip-days 14

can be used to print an event for each third Thursday of each month in
2017.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

contrib/calcurse-dateutil.py

@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+
+import argparse
+import datetime
+
+
+def get_date(s):
+    return datetime.datetime.strptime(s, '%Y-%m-%d').date()
+
+
+parser = argparse.ArgumentParser('calcurse-dateutil')
+parser.add_argument('--date', type=get_date, action='store', dest='date')
+parser.add_argument('--range', type=int, action='store', dest='range')
+parser.add_argument('--unique', action='store_true', dest='unique')
+parser.add_argument('--append', type=str, action='store', dest='append')
+parser.add_argument('op', nargs='+')
+args = parser.parse_args()
+
+
+def skip_days(d, n):
+    return d + datetime.timedelta(days=n)
+
+
+def skip_months(d, n):
+    return d + datetime.timedelta(months=1)
+
+
+def next_weekday(d, w):
+    return skip_days(d, (w - d.weekday() + 7) % 7)
+
+
+def bow(d):
+    return skip_days(d, -d.weekday())
+
+
+def bom(d):
+    return d.replace(day=1)
+
+
+def eow(d):
+    return skip_days(bow(d), 6)
+
+
+def eom(d):
+    return skip_months(bom(d), 1)
+
+
+s = args.date if args.date else datetime.date.today()
+r = args.range if args.range else 1
+a = args.append if args.append else ''
+seen = set()
+
+for i in range(0, r):
+    d = skip_days(s, i)
+    it = iter(args.op)
+    for arg in it:
+        if arg == 'bow':
+            d = bow(d)
+        elif arg == 'bom':
+            d = bom(d)
+        elif arg == 'eow':
+            d = eow(d)
+        elif arg == 'eom':
+            d = eom(d)
+        elif arg == 'next-weekday':
+            d = next_weekday(d, int(next(it)))
+        elif arg == 'skip-days':
+            d = skip_days(d, int(next(it)))
+        elif arg == 'skip-months':
+            d = skip_months(d, int(next(it)))
+    out = "{}{}".format(d, a)
+    if args.unique:
+        if d not in seen:
+            print(out)
+            seen.add(d)
+    else:
+        print(out)