Code review

- enable web fonts in stylesheet
- refactor conversion and rendering code
  * allow XHR to work for local files (status = 0)
  * prevent flash of unrendered content
  * use clearer ipc message name
  * improve method names
  * better error message when XHR fails
- bump max Firefox version
- add dollar sign before command in README
- set safe to server by default to enable includes
- set implicit attributes (no need to disable toc)