- `walk` methods are added to `Block` and `Inline` values; the methods
are similar to `pandoc.utils.walk_block` and
`pandoc.utils.walk_inline`, but apply to filter also to the element
itself, and therefore return a list of element instead of a single
element.
- Functions of name `Doc` are no longer accepted as alternatives for
`Pandoc` filter functions. This functionality was undocumented.
The implicitly defined global filter (i.e. all element filtering
functions defined in the global lua environment) is used if no filter is
returned from a lua script. This allows to just write top-level
functions in order to define a lua filter. E.g
function Emph(elem) return pandoc.Strong(elem.content) end
