Update the way we create stylesheets/load the font data

[Pomax]

There some quirks around font loading in terms of creating the @font-face stylesheet prior to running the loadFont instruction, so it'd be nice to clean that up:

• offer a way to control whether or not to even build an @font-face stylesheet
• offer a way to control whether stylesheet generation errors are terminal or not
• update the README to include those options
• set up dual verification:
  • the @font-face code should guess the font's format based on the URL only
  • the actual bytecode parser should look at the header byteprint to verify we're dealing with supported fonts.