HTML output support

[jcotton42]

Just formalize the HTML dumping option as an actual thing.

• table of contents (pandoc's --toc may do this)
• one file for entire story
• one file per chapter (don't forget to handle the table of contents)
• buttons for changing text size, line height, line length

And inline styles.css, make sure it's not reading it directly, but is instead being passed, so custom CSS (#26) works.