Integration with elm/time

[aecay]

Since elm/time is now the recommended core library for representing time values, it would be nice for the ecosystem if this library integrated with it. The places I can see where integration would be possible are:

• make Html.Attributes.datetime accept a Posix
• let the value of inputs where type is "time" or "date" be a Posix (probably this requires a new function H.A.timeValue. Also make the value passed to the onInput event of those elements be a Posix.

As a stopgap (not requiring any breaking API changes), it would be nice to have functions posixToDom(Date|Time), domDateToPosix, and domTimeToMillis that would convert between the strings that the DOM natively uses and useful elm values.