add render method

lib/DB/IdrisPost.rakumod

@@ -16,3 +16,9 @@ has Bool:D $.idris = True;
 method title(--> Str:D) {
        markdown-title($!source)
 }
+
+# Use katla to highlight our file, mangle the resulting output, and then pass it
+# through to pandoc for html generation
+method render-html(--> Str:D) {
+    die "Not implemented";
+}