Add ability to distinguish non-ISO-8601 datetimes from strings

[alexanderdean]

We could output a warning for these, or a custom date-time format.

• http://labix.org/python-dateutil#head-b95ce2094d189a89f80f5ae52a05b4ab7b41af47
• http://stackoverflow.com/questions/15010210/automatic-date-time-parser-without-specifying-format
• http://stackoverflow.com/questions/3307330/using-joda-date-time-api-to-parse-multiple-formats

Lower priority than #1

[chuwy]

So we should only output warning or also try to guess?

[alexanderdean]

Good question. I think we should pick one or two credible date-time formats which are likely to be widely used and add custom formats for them. I reckon to start with we should only support yyyy-MM-dd as a custom format as that's simple, unambiguous and probably quite widely used...

[christoph-buente]

In order to generate the schema for shopify webhooks, i'd like to have the enum-set for date stringISO 8601 implented. It's widely used, see here: https://docs.shopify.com/api/reference/order