Add a DateTimeWidget component

[n1k0]

UI could be simply 6 select inputs for setting year, month, day, hour, minutes and seconds values. Or alternatively a single text input with some supplementary bidirectional format transformation.

Note that the legit format received from and exposed to formData should always be the JSON compatible date format (eg. new Date().toJSON()).

[n1k0]

This is blocked by tdegrunt/jsonschema#169 as the date-time schema type doesn't seem to be supported by the jsonschema lib we use.

[n1k0]

My mistake, date-time is a JSONSchema string format, not a type. Unblocked.

[n1k0]

Actually JSON datetime format being a perfectly valid format, we can definitely start with implementing support for a simple text field, expecting users to enter a valid JSON datetime string.

We shall iterate to improve the user experience later on, the first priority here being to support the format keyword for datetimes first.