support formatting special events

This is usefull to include events like holidays or semester end in
the formatted output.

I use the following config

```toml
[special_events.Notenschluss]
pattern = "Notenschluss*"
fmt = '''\\rowcolor{{yellow!30}} \\multicolumn{{5}}{{c}}{{{event.start.date().strftime('%d.%m')} {event.summary}}} \\\\'''

[special_events.Ferien]
pattern = "*ferien*"
fmt = '''\\rowcolor{{yellow!30}} \\multicolumn{{5}}{{c}}{{{event.start.date().strftime('%d.%m')}-{event.end.date().strftime('%d.%m')} {event.summary}}} \\\\'''
```

to include any holidays as well as the semester end in my formatted
latex table.

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