Retry conversion by passing a string instead of sources when the
`Reader` fails with a message that hints at an outdated function. A
deprecation notice is reported in that case.
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.
