Support costom note types

[nicolo-ribaudo]

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;
   }