Add footnote and note numbering to Tables

[SteveLLamb]

Closes #374, closes #375

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[thomasheritage]

Thanks a lot @SteveLLamb -- from a quick test this seems to be working well for notes and footnotes in Tables.

A couple of things:

• In testing, I edit Table 1 to include a reference to a footnote defined in Table 2. This should be prohibited. In the rendering, Table 1 includes <sup><a href="#fn-doc-example"></a></sup> -- that is, the html for the link has been included but is empty and so is invisible. No in-browser error is displayed.
• Now that "notes to tables" are supported, it would be useful to provide some guidance in the html-pub documentation about where to put them -- I assume that a table-wide note should go into <tfoot><tr><td> i.e. alongside any footnotes. Perhaps the footnotes example in "7.4.10 Tables" could be updated to also include a table-wide note alongside the footnote? (in fact, any Notes have to come just before footnotes according to ISO -- see this example)

[SteveLLamb]

@thomasheritage ah yes, cool, have only <tfoot> validation on footnote, but i need to add in "self containment" validation too.

I will look at more guidance on the documentation for notes as well.

Great feedback.