to support single file runs

TODO

@@ -129,3 +129,19 @@ cleanly create and teardown temporary files.
 
 On the other hand, this could just be sourced explicitly in the test file,
 without the special setup and teardown feature.
+
+Run on a file
+----------------
+Presently you can run urchin only on a directory.
+It would be neat if you could run it on a file as well.
+
+This occurred to me when I wanted to run
+
+    urchin test/fast/Unit\ tests/nvm_ls_current
+
+on the nvm tests. I wound up running this instead.
+
+    urchin test/fast/Unit\ tests/ | grep nvm_ls_current
+
+The Molly guard would be assessed, and the corresponding setup, setup_dir,
+teardown, and teardown_dir files would be run in the appropriate order.