It's a little hard to style the current ERDs. This aims to make things
easier by associating a distinct class with each type of element within
the diagram.
I've added a common `er` class across the elements so that those that
bring their own `themeCSS` can target styles at this diagram type. This
hasn't really been done elsewhere so I'm open to suggestions; an
alternative may be to add classes to a top-level element so that we can
do e.g. `svg.mermaid.er .entityBox`.
