Add sourceDate property

[mattgarrish]

This pull request adds the property for expressing the date on which the source was published.

Should we also move this and sourcePublisher to the recommended properties list, as I asked in #221? Looking at the sourcePublisher definition, it says to add "N/A" if there isn't a source publisher, which is a strong case for not making it required.

Required metadata should always have a meaningful value, otherwise we keep having to propagate a workaround. It's never great to require language-specific values like "N/A", which I'll have to work into the date if we keep them required.

Fixes #200

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[clapierre]

I am concerned about the prefix "a11y" as these are not accessibility metadata.
Is there something more appropriate?

[mattgarrish]

The source publisher and source publication date are pretty unique to accessible format production and we're using a11y as a prefix for metadata that is broadly needed for accessible formats. I wouldn't try to read too much into a prefix.

We're also constrained by epub's reserved prefixes. If we mint something new, then the prefix has to be declared every time until we can convince the epub wg to adopt it as a reserved prefix (and then get the spec updated, and then get epubcheck updated...).