Avoid highlighting of "strings" by deriving from
`text-mode-syntax-table`. This fixes missing highlighting in default.html5
tag attributes.
Define comment starting and ending sequences to allow use of M-q (auto-fill)
on comments.
The pandoc module documentation in doc/lua-filters.md was automatically
generated from `data/pandoc.lua`. A make target is provided which uses
a lua filter to update the documentation.
