Added changes to ims_lti_py/tool_config.py to allow for sub options.  ex...

...amples of this can be found at https://canvas.instructure.com/doc/api/file.tools_xml.html.

Moved NSMAP out of the class method so it could be reused by multiple methods.

Added a test in tests/tool_config_test.py to confirm that adding a sub option results in the desired XML config fragment