Provide method for metadata processing in lua filters

[tarleb]

Allow processing of metadata without having to push the whole document to the lua stack. Additionally, the pandoc.reader lua submodule is removed in favor of a simple read function which takes a string and format specifier as arguments.

Additional API change: All functions defined directly in the Text.Pandoc module are pushed to Text.Pandoc.Extensions or one of the newly introduced Text.Pandoc.Readers or Text.Pandoc.Writers modules. This is technically not necessary.

This change is intended as a solution to #1960. The following uses the metadata from metadata-file.yaml.

return {
  {
    Meta = function(meta)
      local metafile = io.open('metadata-file.yaml', 'r')
      local content = metafile:read("*a")
      metafile:close()
      return pandoc.read(content, "markdowns").meta
    end,
  }
}

[jgm]

@tarleb
These look fine to me, so go ahead and merge them.

By the way, at some point we'll need a tutorial on how to use the lua filters.