Updates to use figure HTML tag for tables and figcaption for table
captions.
Updates image rendering to use figure/figcaption.
Adds styling to table caption.
Removes quotes from table captions.
Adds rudimentary figure numbers in captions.
Adds chapter index option and uses it to mark figures and tables.

Chapters get an :index property that takes a string that is prepended to
figure captions to mark them as part of the book as a whole. Chapter 1
will have an index of "1" by default through the Book class, which means
figures are 1-1, 1-2, etc.

Added references of the form @[tag] where tag is given by the figure or
table and then can be used anywhere throughout the chapter.

Images use this syntax to provide a tag (named 'tag'):

![Image Alt](tag@file.png "Title Text")

Tables use this syntax to provide a tag (named 'tag'):

!table tag "Table title"
Adds images property to Chapter and MarkdownRenderer.

This property gives you a list of all images used within the chapter.
Adds images property to Book.

This property will collect all images used throughout the book.
Adds functionality to generate a page for table of figures.