Add make target to automatically regenerate html fixtures

After changing something the fixtures need to be updated to reflect the new
output. This just moves those steps from my bash history to make.

etc/generate-html-fixtures

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+for org_file in org/testdata/*.org; do
+    ./go-org $org_file html > org/testdata/$(basename $org_file .org).html
+done