Add non-strict parsing mode to avoid non-standard ICS files to break parsing

[filecage]

This is a (late) followup to #1: the strictness of the parser causes it to be useless in many real-life scenarios because there are a lot of non-standard conform iCal implementations out there. Also, there are extensions to standards that are not supported by this library.

Best is to let userland choose between parseString() and parseStringNonStrict().