add short form field to annotation

[jacobagilbert]

Per discussion in #120 and elsewhere, there is no short-form machine readable core field, and many situations where this is valuable.

closes #131

[Teque5]

Any changes to the spec should also update the schema.

[jacobagilbert]

@Teque5 I updated the spec and changed the field name to label instead of description since I agree with your other comment. I put may/should language describing the 20 character recommended limit; i don't think it makes sense to force this but I agree it is a good idea to provide some guidance for 'best' compatibility.

[bhilburn]

Left a question re: removal of whitespace semanticism, requested change on label spec text.

Thanks for filing this, @jacobagilbert!