Improve strict mode and error collecting

[kewisch]

In #186 we added a strict/lenient mode for the parser. This is a good first step for making things more configurable, but it isn't great overall. While implementing I also noticed that "strict" mode isn't actually that strict. There are various length checks being skipped and the converted values are not actually validated.

One possible approach would be to have icalSet in the design data be the lenient set, and add a strictIcalSet that does additional checks, including all necessary length and data type checks. The lenient one would accept a date as date-time and vice versa. You'd then be able to pass a specific set to ICAL.parse. Another approach might just be a strict flag on ICAL.parse, that would select the set automatically.

Then again, some folks might want lenient mode, but not as lenient that it would accept all types. Another idea might be an error collector that would have a function that would be able to resolve certain errors. One provided resolver might be one that swaps date/date-time accordingly.