Add support for custom diagram styles via diagram comments

[eFrane]

Since the component default-slot syntax is deprecated in 2.x,
support for using custom styling should be re-added.

There are two ways to go here:

1. add argumentsto the fenced-block language descriptor, e.g. ``` mermaid [style={css;rules;here;}]
2. parse the mermaid content for a special comment syntax, e.g. %% style css: rule;

I currently tend towards going the second route as it seems more robust
and also wouldn't destroy anything if the code blocks were copied
into any other system.