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`.
This restores the option of rendering an ERD with an absolute width,
which is consistent with the approach taken with the other diagram
types.
This logic was lost in #1324, which was probably just a small oversight:
9199546dca (diff-7c38d27acbe0676d923bf19283671616L397-L409)
The option defaults to true for backwards compatibility.
