Unspecified digits incorrectly formatted

[1ec5]

When a date contains unspecified digits, the format() function produces all X’s. For example:

> edtf.format(edtf.default('XXXX-04-01'));
// Expected: 4/1/X
// Actual: 4/1/?
> edtf.format(edtf.default('201X'));
// Expected: 2010s
// Actual: XXXX
> edtf.format(edtf.default('20XX'));
// Expected: 21st century
// Actual: XXXX
> edtf.format(edtf.default('20XX'));
// Expected: 3rd millennium
// Actual: XXXX
> edtf.format(edtf.default('XX00'));
// Expected (stretch goal): turn of the century
// Actual: X

[inukshuk]

As mentioned in the other thread, formatting isn't part of the spec so we're pretty much on our own here. So far we're pretty much just adding a 'circa' for approximate dates and adding a question mark for uncertain. If you could add more examples like those above we can take this further.