Parse :root pseudo class and change parse example usage to match help text

[Zentropivity]

• parse :root pseudo class
• examples/parse now works as expected from usage help text: parse - 'p { color: red }' (instead of piping input in through stdin)
  can also use it like this: cargo run --example parse -- - 'p { color: red }'
• parse css variables like --color: #123456 by allowing names to start with -