Support costom note types

In ECMA-426, we have more types of "notes" than the two supported by ecmarkup. For example, in http://tc39.es/ecma426/#sec-linking-inline you can see an "Example" and an "Issue".

We currently make them work by using <emu-note example>/<emu-note issue> with some custom CSS, but we have to inject the "Example"/"Issue" title through CSS so it's not user-selectable.

It'd be great if Ecmarkup:

1. allowed custom values for emu-note's type attribute, and used it to generate the label and to maintain the counter, type="editor" would be special-cased to have "editor's note" rather than just "editor" as the label
2. the various colors were customizable through CSS variables, so that we could simply do

   emu-note[type=example] {
     --note-text-color: #574b0f;
     --note-border-color: #e0cb52;
     --note-background-color: #fcfaee;
   }
   

Review ecmarkup's current emu-note handling and compare it with ECMA-426's spec.emu lines 26–55, which demonstrate the existing CSS workaround. Done means custom note types produce selectable labels and counters, editor notes use the requested special label, and note colors can be set through CSS variables.