Return null date[time]s if unparseable

[colmanhumphrey]

Fixes #14

Previously, if all the patterns failed, the default was to return the result of parseISO. But that could also fail to parse, returning "Invalid Date".

So now this also checks if that final value is valid, and if not, returns null.

Question: should it be null, or undefined, or doesn't matter?

Other stuff (minor):

• Added some null/undefined values to the default example/data.js for quick viewing, also added a plaindate type here too
• Early return: once a valid pattern is found, parseDatetimeString doesn't then loop through the remaining pattterns

Happy to do any other edits here too, e.g. let me know about version numbers etc

[Wattenberger]

this is great, thanks @ColmanHumphrey!