global: license change to Revised BSD License

* The Flask-Menu extension is now released under more permissive
  Revised BSD License.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
docs: richer quick-start usage example

* Expands canonical quick-start usage example with menu ordering and
  template rendering examples.  (see #15)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: integration to single page

* Improves PEP8 and PEP257 of all files.

* Integrates documentation on a single page.

* Adds tests for strict Sphinx documentation build and PEP257.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
menu: additional keyword arguments store

* Stores additional keyword arguments as `MenuItem` attributes.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
tests: testing the keyword arguments of Menu

* Tests the keyword arguments and if they cannot be overriden.

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>