Improve code highlighting typography
Improve code highlighting typography

- Don't let user agent stylesheet `pre` override fonts set in `.highlight`
- Don't use ligatures in code samples.
davidpett/guides,skaterdav85/guides,MarkMT/guides,davidpett/guides,skaterdav85/guides,knownasilya/guides,jkarsrud/guides,ubuntuvim/guides,bmac/guides,knownasilya/guides,tleskin/guides,davidpett/guides,davidpett/guides,skaterdav85/guides,bmac/guides,tleskin/guides,nicobot/guides,skaterdav85/guides,ubuntuvim/guides,ubuntuvim/guides,MarkMT/guides,tleskin/guides,jkarsrud/guides,knownasilya/guides,nicobot/guides,MarkMT/guides,jkarsrud/guides,jkarsrud/guides,knownasilya/guides,MarkMT/guides,bmac/guides,HeroicEric/guides,HeroicEric/guides,HeroicEric/guides,bmac/guides,nicobot/guides,ubuntuvim/guides