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, 7 insertions, 0 deletions

diff --git a/pandoc-lua-engine/test/writer-template.lua b/pandoc-lua-engine/test/writer-template.lua
new file mode 100644
index 000000000..c90f7c1ef
--- /dev/null
+++ b/pandoc-lua-engine/test/writer-template.lua
@@ -0,0 +1,7 @@
+function Writer (doc, opts)
+  return pandoc.write(doc, 'gfm', opts)
+end
+
+function Template ()
+  return pandoc.template.compile '<!-- start -->\n$body$\n<!-- stop -->\n'
+end