Update Travis: run unit tests

Author: kinow

Makefile

@@ -24,7 +24,18 @@ watch:
 	@echo "Building and watching for changes in the documentation."
 	sphinx-autobuild --ignore venv . _build/html
 
+## unittest         : run unit tests on checking tools.
+unittest :
+	@bin/test_lesson_check.py
+
+#-------------------------------------------------------------------------------
+# Include extra commands if available.
+#-------------------------------------------------------------------------------
+
+-include commands.mk
+
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+