Add support for recurring events about zerocal

[mre]

I like both. 😉 A mix of both might be nice:

# Repeat every week & for only 4 times, interval is counted in minute
curl -L 'https://zerocal.shuttleapp.rs?start=2022-11-04+20:00&duration=3h&title=Birthday&description=paaarty&repeat=4&interval=1w'

[Vinfall]

Just some random thoughts on the format:

# More like natural language
# Repeat every month till forever, case-sensitive
curl -L 'https://zerocal.shuttleapp.rs?start=2022-11-04+20:00&duration=3h&title=Birthday&description=paaarty&recurrence=MONTHLY'
# More programming-friendly
# Repeat every week & for only 4 times, interval is counted in minute
curl -L 'https://zerocal.shuttleapp.rs?start=2022-11-04+20:00&duration=3h&title=Birthday&description=paaarty&repeat=4&interval=10080'

I guess these two can work together to make the format more robust. Or maybe just use something like repeat=0 for one-time event and repeat=FOREVER/repeat=-1 for events recurring forever.