The first argument passed to Lua `Reader` functions is no longer a plain
string but a richer data structure. The structure can easily be
converted to a string by applying `tostring`, but is also a list with
elements that contain each the *text* and *name* of each input source as
a property of the respective name.
A small example is added to the custom reader documentation, showcasing
its use in a reader that creates a syntax-highlighted code block for
each source code file passed as input.
Existing readers must be updated.
This is better as an example. And it is faster than pandoc's
regular creole parser, which shows that high-performance readers
can be developed this way.
