Lua: support extensions in custom writers

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,
    }

1 files changed, 12 insertions, 0 deletions

diff --git a/pandoc-lua-engine/test/extensions.lua b/pandoc-lua-engine/test/extensions.lua
new file mode 100644
index 000000000..cea9a45a1
--- /dev/null
+++ b/pandoc-lua-engine/test/extensions.lua
@@ -0,0 +1,12 @@
+function Writer (doc, opts)
+  local output = 'smart extension is %s;\ncitations extension is %s\n'
+  local status = function (ext)
+    return opts.extensions:includes(ext) and 'enabled' or 'disabled'
+  end
+  return output:format(status('smart'), status('citations'))
+end
+
+writer_extensions = {
+  smart = true,
+  citations = false,
+}