add support for modeline

[dmalan]

• Recognize below, where ... is any freeform string (for now).
  • # cs50: ...
  • // cs50: ...
  • /* cs50: ...
• Can appear in first line or, hashbang, second.
• Ignore when comparing files.
• Remove from output, display value thereof elsewhere in UI.
• If present on first line and blank line below it, remove blank line too.