v0.6.0

Some tooling produce full day events in an inclusive manner
(e.g. DTSTART=20200101 and DTEND=20200101) which goes against RFC. This
version expands those events to span the day instead of being immediate.

The StrictMode attribute has also been moved into a dedicated struct, to
allow for future additional strict settings. This is a breaking change
from the previous version.

Added URL attribute.

Added support for non-standard properties (X-*).

Fixed tests. Filter events with no start or end date.

0.2: support for RRULEs