Migrating this repo to pandoc/lua-filters

[ickc]

@tarleb, @rohieb, @jperon, @majorgreys, @cagix, @tie,

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It is unfortunate that the concept of having a centralized lua filter repo has 2 different realizations because one existed a week before the other and likely the other hasn't realized the existence of this one yet.

In short let's focus the effort on one of them only, and it is obvious that pandoc/lua-filters is more active. Let's migrate to there.

See pandoc/lua-filters#30

Edit: it means this one will be deleted once the migration is done. As long as all filters here are migrated there I will delete this.