Format attributes

[KayEss]

It isn't totally clear that a validator should ignore any format attribute it can't check/doesn't understand. The closest the wording comes is in 7.2:

Implementations MAY add custom format attributes

I think this implies that the validator is likely to come across format attributes it won't understand, but it probably ought to be explicit. Maybe something like:

A validator MUST ignore any format attribute which it cannot check.

Or under the other two points:

they SHOULD ignore any attribute value they don't validate

[vearutop]

santhosh-tekuri/jsonschema#21 here is a relevant issue for one of validators.

[handrews]

I'll deal with this at the same time as #646.