Generate HTML version

Motivation

Previous HTML generation was broken.

Modifications

Use htlatex with the following tweaks:

- Perform additional image conversion so that no images are omitted
- Prefer <em>, <b>, and <code> to weird CSS classes
- Tweak the default CSS
  - Use Google Web Fonts
  - Adjust line height, margin, and padding
  - Fix formatting of lstinline
- Remove unused scripts and targets
- 'make all' now builds perfbook_html by default

Result

A beautiful single-page HTML