Pandoc lua filters support

[srid]

Emanote definitely should support lua filters so that users can include these filters in their notebook along with .md and .tpl files.

You should be able to apply these filters on per-note or per-folder basis via specifying them in .yml files. Just like you can specify a .tpl template.

Originally posted by @srid in #250 (comment)

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• MVP version: [MVP] Basic Pandoc lua filters #278
• Final version

[srid]

We can call Text.Pandoc.Filter.Lua.apply in the patch function, but this is not enough, because updating the filters themselves won't trigger the re-patching of all Pandoc documents.

Alternatively, we can apply the filter during the rendering stage - but apply is an IO action, and this will be expensive. It still won't solve the problem of automatically updating dependent docs (like backlinks).

Requires some thought here ...

[srid]

Proof of concept: https://gist.github.com/srid/6e17e3ad68a3f6ba877a8024b8ea0909 w/ git clone https://github.com/pandoc/lua-filters.git ./default/lua-filters.

That gave me an idea that we should include https://github.com/pandoc/lua-filters by default in the 'default' layer. EDIT: But this can only be done after adding a mechanism to exclude folders from being copied over to the static site.