Adds tables property to Book and renders Table to Tables page.

Adds tables property to MarkdownRenderer, Chapter, and Book. This
property will collect the tables per chapter and the aggregate them
together in the Book. The book layout engine then renders the table of
tables.
Adds missing tests for parse_table in MarkdownRenderer.

Fixes a bug when producing a tag when no index is passed to the
renderer.
Do not output table of tables/figures if there are none.
Ensure that the chapter header is printed when given.
Ensure <strong> and <em> tags are wrapped in <p>.
Adds ability to create prefaces.