Add test for --source flag and pull request suggestions

Add a demo/sourcing.vars file to be used by the associated demo
script.

Modified mo argument parsing so that --help and --source=file do not need
to be the first arguments for mo. And can in fact appear after or in the
middle of filenames.

tests/source.expected

@@ -0,0 +1,5 @@
+value
+* 1
+* 2
+* 3
+AAA BBB