[API Change] Base custom writers on Writer type.

The `T.P.Lua.writeCustom` function changed to allow either a TextWriter
or ByteStringWriter to be returned. The global variables
`PANDOC_DOCUMENT` and `PANDOC_WRITER_OPTIONS` are no longer set when the
writer script is loaded. Both variables are still set in classic writers
before the conversion is started, so they can be used when they are
wrapped in functions.

data/sample.lua

@@ -13,19 +13,19 @@
 -- produce informative error messages if your code contains
 -- syntax errors.
 
+function Writer (doc, opts)
+  PANDOC_DOCUMENT = doc
+  PANDOC_WRITER_OPTIONS = opts
+  loadfile(PANDOC_SCRIPT_FILE)()
+  return pandoc.write_classic(doc, opts)
+end
+
 local pipe = pandoc.pipe
 local stringify = (require 'pandoc.utils').stringify
 
--- The global variable PANDOC_DOCUMENT contains the full AST of
--- the document which is going to be written. It can be used to
--- configure the writer.
-local meta = PANDOC_DOCUMENT.meta
-
 -- Choose the image format based on the value of the
--- `image_format` meta value.
-local image_format = meta.image_format
-  and stringify(meta.image_format)
-  or 'png'
+-- `image_format` environment variable.
+local image_format = os.getenv 'image_format' or 'png'
 local image_mime_type = ({
     jpeg = 'image/jpeg',
     jpg = 'image/jpeg',