Table tags are no longer self-closing

Table tags no longer self-closing. Rows and cells now contain their
contents. Also refactored out an `emit_table_tag` method.
Note: this will require changes to the Tag node and possibly the builder,
those changes will be in the next commit.
Change wiki tags to use style separators

For wiki syntax tables, add `wiki_style_separator` as an attribute
for the Tag node. Also reorder `closing_wiki_markup` property and tests
to match its place in the constructor.