Add styling for code highlighting

This adds styles for deleted lines and for highlighted (or new) lines.

Note that switching sub-styles in the middle of a code block will
swallow any leading blank line before, so in some cases you'll see two
blank lines in the tilde block where there would normally be one. This
will still render with a single blank line.

In addition to addressing existing cases, I've also found other cases
where highlighting illustrates incremental changes from one version to
the next, particularly in _Ray Tracing In One Weekend_ (book 1).

See issue #14
Resolves #11

Author: hollasch