Allow MenuItem url to be callable

[timnyborg]

Treats url the same way as title, children. Handles cases like #70 without the need for subclassing.

[codecov]

Codecov Report

Merging #71 (d753e95) into master (5b253e6) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   95.83%   95.98%   +0.15%     
==========================================
  Files           5        5              
  Lines         312      324      +12     
==========================================
+ Hits          299      311      +12     
  Misses         13       13              

Impacted Files	Coverage Δ
menu/menu.py	96.94% <100.00%> (+0.04%)	⬆️
menu/tests/test_menu.py	100.00% <100.00%> (ø)

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[jokiefer]

Looks good to me.
Maybe an explicit example for generating links to object based views would be nice, to avoid missunderstanding the feature of callable url attribute.

[kytta]

Great job! Nothing to change in code from my end. Could you please update the branch to the latest state of master so that the needed CI checks can run?