Remove highlight.js in favor of Hugo's Chroma syntax highlighter.

[cedws]

This pull request addresses #109 by using Hugo's Chroma compile-time syntax highlighting introduced in version 0.28.
There are two potentially breaking changes:

• pygmentsUseClasses must be removed from the config.toml, otherwise the pygmentsStyle option will be ignored be Hugo.
• pygmentsCodeFences must be configured to true, and pygmentsStyle must be set to use one of the themes here.
  Hugo still uses the pygments entries with Chroma for compatibility. Find out more here.
  Let me know if you have any questions or suggestions .

[cedws]

Closing this PR due to inactivity.