Add support for cssClasses

[hungsu]

Mentioned in #9

Obsidian allows users to add a YAML entry, cssclass, which applies a CSS class to the current note.

Typomagical's existing Style Settings classes are ideal candidates for this. But we would need a few things:

• [ ] Modify stylesettings classes so that they work for both stylesettings and also the cssclass meta
• [ ] Create easily accessible and readable documentation to teach users what cssclasses they may use and what those classes do