New module Text.Pandoc.Readers.Custom, exporting
readCustom [API change].
Users can now do `-f myreader.lua` and pandoc will treat the
script myreader.lua as a custom reader, which parses an input
string to a pandoc AST, using the pandoc module defined for
Lua filters.
A sample custom reader can be found in data/reader.lua.
Closes#7669.
