Add support for nested tables

Asciidoc supports tables in tables, but you need to be able to specify the column separator and table beginning/end specifier (by default these are | and |=== respectively) for the nested table (according to the asciidoc docs, you change them to ! and !=== respectively). I think this could be done by adding these to the table properties when creating the table or by providing accessors that allow them to be "overwritten" in code if desired. That way no other class is required, the end user can simply modify the separators if a table is going to be rendered into a cell in another table.