When writing a highlighter, pass in a Content -> Content function on init.
The function should modify the highlightClasses or highlightStyling fields of the elements.
Each string x in highlightClasses turns into Html.Attribute.class x.
Each (x,y) in highlightStyling turns into Html.Attribute.style x y.
Attributes of LineBreaks apply to the preceding paragraph as a whole.
For toolbar icons or shortcut keys. See the example for tips.