Provides appendix support

This provides support for single and multiple appendices (fixes issue #21).

On the first pass, we assume there is only one appendix and therefore don't give it a letter. On the second pass, we fix the heading, giving it a letter if there are multiple appendices. On the third pass, we fix the table of contents.

To make use of appendices, just write `\appendix` and then use the `\chapter` macro for each appendix heading as usual.