Support for compound

[exavolt]

The type is string but the value is actually a compound of two or more fields.

For example measurement is a compound of the value and the unit. The output will be something like 25 m.

Every subfield could be defined independently. For example, the value is limited to 0 to 999 and the unit could be meter, feet, parsec. The delimiter is configurable too.

Another example is URL. It's constructed from elements: scheme, host, path, query, fragment.

Date-time is another obvious example.

See date-time editor on OS X for the reference.