Number tables

[bwhelm]

As the first commit indicates, this allows for table captions with attributes. It seems to work reasonably well for LaTeX, html, and docx.

Notice that latex_table() is a big hack. I'd appreciate better ideas for how to insert an appropriate \label{id} command into the LaTeX generated by pandoc.

[aaren]

Table referencing was something that I meant to add, but didn't get round to.

This solution is a great hack and makes sense given the current limitations of the code. However, it might be possible to do some higher level restructuring to make this a bit more maintainable.

Also, would be good to keep compatibility with pandoc-crossref tables in terms of syntax.