Lua: Support built-in default templates for custom writers

Custom writers can define a default template via a global `Template`
function; the data directory is no longer searched for a default
template.

Writer authors can restore the old lookup behavior with

``` lua
Template = function ()
  local template
  return template.compile(template.default(PANDOC_SCRIPT_FILE))
end
```

1 files changed, 16 insertions, 0 deletions

diff --git a/doc/custom-writers.md b/doc/custom-writers.md
index 038067a3e..aa2796d00 100644
--- a/doc/custom-writers.md
+++ b/doc/custom-writers.md
@@ -83,6 +83,17 @@ function Writer (doc, opts)
 end
 ```
 
+## Default template
+
+The default template of a custom writer is defined by the return
+value of the global function `Template`. Pandoc uses the default
+template for rendering when the user has not specified a template,
+but invoked with the `-s`/`--standalone` flag.
+
+The `Template` global can be left undefined, in which case pandoc
+will throw an error when it would otherwise use the default
+template.
+
 ## Example: modified Markdown writer
 
 Writers have access to all modules described in the [Lua filters
@@ -106,6 +117,11 @@ function Writer (doc, opts)
   }
   return pandoc.write(doc:walk(filter), 'gfm', opts)
 end
+
+function Template ()
+  local template = pandoc.template
+  return template.compile(template.default 'gfm')
+end
 ```
 
 [Lua filters documentation]: https://pandoc.org/lua-filters.html