Add format "duration"

[ralfhandl]

Please add a format for durations in the format defined by ISO 8601 and cited by RFC3339 and XML Schema 1.1.

Example values:

• P15DT1H22M1.5S fifteen days, one hour, twenty-two minutes, one and a half seconds
• P30D thirty days
• PT5M five minutes

[awwright]

Since we've already got a bunch of other RFC3339 productions, I don't think this is unreasonable.

[handrews]

@awwright @ralfhandl PR welcome :-)