Add support for writing to custom formats

[erikc5000]

It should be possible to define a custom format similar to how a custom parser can be defined. Something like:

val customFormat = dateTimeFormat {
    monthNumber()
    +'/'
    dayOfMonth()
}

val date = Date(2019, 10, 28)
val string = date.toString(customFormat)

For the initial implementation, we can avoid adding support anything locale-specific. Locale will open up another can of worms.

Implementing this will probably require inserting an interface into the date-time primitives -- something akin to TemporalAccessor in java.time, but Period and Duration should also implement it since we include parser support for them. This interface should provide access to the available date-time fields on the object.

interface Temporal {
    fun get(field: DateTimeField): Long
}

Introducing a TemporalQuery-like interface for access to non-Long fields may also be necessary.

Ultimately, I'm envisioning the abstractions added here will also tie in with and enable parsing of arbitrary ISO-compliant strings.

val temporal = genericIsoParser.parseAny(text)

if (temporal is Year) {
    // ..
} else if (temporal is Date) {
    // ..
}

This will be useful for working with intervals that have mixed-precision end points, like "2018/2019-05-01".

Obviously, these are more advanced use cases though, so the focus should initially be on what's necessary to support custom formatting.

[erikc5000]

An effort is add formatting support is well underway. It's a pretty big change and will break the existing parser functionality to some extent.