Add __init__.py to packages.

This is required for my particular use case -- running pytest on the
generated code directory -- due to
<pytest-dev/pytest#774>. It ought to be
abstracted out, especially if we ever support non-python code blocks.

mkcodes.py

@@ -110,6 +110,8 @@ def main(inputs, output, github, safe):
         outputdir = os.path.dirname(outputfilename)
         if not os.path.exists(outputdir):
             os.makedirs(outputdir)
+            with open(os.path.join(outputdir, '__init__.py'), 'w'):
+                pass
 
         with open(outputfilename, 'w') as outputfile:
             outputfile.write(code)