[Ruby] Release 0.19.0

Added

• remove_inlined_selectors option to remove selectors that were successfully inlined from <style> blocks.

Changed

• Update cssparser to 0.36.
• Update html5ever to 0.36.
• Update selectors to 0.33.

Performance

• Use element indexes for faster selector lookups on large documents.
• Use more efficient data structures for storing element styles.