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>
Flask-Menu v0.2.0

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>