Add CSS to match Pixelpark design

In custom CSS, define:

- Title slide with background image and white text
- Chapter title slide with black-to-white gradient background and
  white text
- Dark grey default text
- Light green headings
- Light green links
Add facility to customize CSS

Invoke pandoc with the --css option so we can apply custom CSS
styling.
Merge remote-tracking branch 'fghaas-hk-pl/master'