Move all untis related configuration into the webuntis section.

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.

I want to color a table row in a certain color if a early end of class
causes my lesson to be canceled.
To do so I use the new configuration table:

```toml
[event_line_formats]
Unterrichtsschluss = '\\rowcolor{{red!30}} {line}'```

When generating latex the '&' separator breaks the resulting table.

I use this oputput configuration:

```toml
output_header = """
Woche Datum     LP-Abschnitt Std. LP-A. Unterrichtseinheit
----- --------- ------------ ---------- ------------------------"""
fmt = "{week:>5} {date.strftime('%d.%m'):>9} {' '*12} {' '*10} {period_events}"
```

To produce a formatted pandoc simple table looking like this:

    Woche Datum     LP-Abschnitt Std. LP-A. Unterrichtseinheit
    ----- --------- ------------ ---------- ------------------------
       16     18.04                          Event1 & Event2
       17     25.04                          Event3
       18     02.05                          Event3 & Event4
    ...