Fixes to date & timezone parsing, add ignore_errors flag

[timsu]

We've seen a number of errors in Outlook-generated ICS files in the wild. This PR adds a few fixes to make the parser more flexible:

• simple fix to support multiple exceptions in EXDATE
• add an options field with a flag to ignore_errors when deserializing to skip events with errors
• simple parsing for custom timezones. this doesn't parse VTIMEZONE properly but at least prevents a crash

[lpil]

This approach seems somewhat heavy-handed to me. Is there a particular kind of error that is expected?

[timsu]

Are you talking about ignore_errors, or custom timezones parsing? The custom timezones I agree it's not great, the ideal would be to actually interpret VTIMEZONE properly. I can remove that part of the handling if you'd like.

This comes from crashes we've seen trying to parse user ics files, mostly generated by Outlook 365. I can also attach some samples if that's helpful.

[lpil]

The ignoring of errors part. I would much rather handle it without swallowing exceptions. Is there a particular bit that is expected to fail?