1.15

Many of the changes in this version are due to Linwan Song.

• Added support for Standard ML (sml) language syntax highlighting.
• Updated to highlight.js version 11.6.0.
• Updated PyxlScript syntax highlighting rules.
• Patches for unclosed HTML tags.
• Support for single-column image grids.
• Support for YouTube videos with timestamps.
• Fixes for Diagram captions