Add a 'multiple' property to options

[evert]

Mentioned in a few different threads, but I think it deserves its own issue.

When an 'options' property is present on a property, it potentially allows the selection of more than one item.
In JSON, if multiple are selected, this gets encoded as an array of strings.

It would be helpful for our implementation if we can add an additional multiple property on the options object.

If multiple is set to true:

• The value will always be encoded as an array. The array can have 0 or more properties. 0 if none is selected.
• minItems and maxItems are allowed to be set.
• value and selectedValues should be an array.

If multiple is set to false, or omitted (the default)

• The value will be encoded as a string.
• minItems and maxItems are ignored/forbidden.
• • value and selectedValues should be a string.

[mamund]

seems reasonable. can you work up a HAL-FORMS example for review?