Use HTML custom data attributes for counters, etc.

[sujato]

One of the very few outstanding issues with the Bilara Pali texts is the presence of some small bits of helper information. Namely:

• counters (found widely)
• right/wrong view passages in Kathavatthu
• forward/reverse pasages in Yamaka

I have discussed this and made proposals previously here:

suttacentral/bilara-data#508
suttacentral/bilara-data#230
#1598

This supersedes all previous discussions.

Rather than create a new data directory (heavy!) or use CSS classes (clunky!) let's use HTML data- attributes. That what they're for!

I have added three kinds of attribute:

• data-counter
• data-doxy
• data-direction

And display is activated using .highlight in CSS only (no JS). It works great!