Add support for UTC parsing mode

[Wenzil]

When coercing a date string omitting the time part (e.g. YYYY-MM-DD), it was previously always parsed in local time. For example in the EST time zone, 2018-01-01 parsed to 2018-01-01:05:00:00.000Z.

This PR adds an optional utc mode to Joi.date() coercion. Usage:

const schema = Joi.date()
  .utc()
  .format('YYYY-MM-DD');
schema.validate('2018-01-01');

This parses to 2018-01-01:00:00:00.000Z regardless of local time zone

[lock]

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