Custom writers can define the extensions that they support via the
global `writer_extensions`. The variable's value must be a table with
all supported extensions as keys, and their default status as values.
E.g., the below specifies that the writer support the extensions `smart`
and `sourcepos`, but only the `smart` extension is enabled by default:
writer_extensions = {
smart = true,
sourcepos = false,
}
John MacFarlane