User-configurable colours for categories

[martindholmes]

Colours for categories are currently assigned using an algorithm based on a set of ten pairs of colours/background colours which are designed to be maximally distinctive without clashing. However, it would also be useful to allow colours for specific categories to be configured in the TEI file; specific colours might match the purpose of categories in particular projects.

This could be achieved by using @style on the <category> element and providing color and background-color properties; those could then be harvested into the JSON, applied as custom properties to the category, and then passed to the getCategoryStyle function.

[martindholmes]

One worry here would be that @style is specifically defined as capturing the appearance of a source text. There is (usually) no source text here, and the use of @style is arguably tag abuse. I'll think about some more appropriate approaches here.