MenuItem's kwargs should be checked to be callable

[nerdoc]

As stated in #78, MenuItem's __init__() adds all additionally added kwargs as instance variables. This is great for new args lico icon etc., as long as they are "static", meaning they do not change over time in the template's context. There are many variables I can think of (even icon!) like

• badge (which should return e.g. a number of new emails), or
• disabled (which should be true if a certain condition is met)
• ...

where you would need to add it to MenuItem, and it would be easy with kwargs, but MenuItem doesn't check if it is callable.

So I would propose to

1. put kwargs in a dict self.kwargs to have a reference later
2. nstead of making reference vars of them in the first place, do this after 1., to hold the same reference. Please correct me if this is not correct in Python... (untested, just as idea here)

def __init__(..., **kwargs)
    # ...
    self.kwargs = kwargs
    # keep a link of all kwargs in the instance, for convenience/template context
    for k in self.kwargs:
        setattr(self, k, self.kwargs[k])    

def process(self, request):
    # ...
    # if kwargs are callable, call them and save the results in the actual instance variables
    for key, value in self.kwargs:
        if callable(value):
            setattr(self, k, self.kwargs[k]())    

[nerdoc]

I don't want to create another issue: This should also be applied to the url attr, which is a "core" attribute and not dynamicaly generated using kwargs: url should be potentially callable too.

use case: I want to point to a URL like /person/<int:pk/edit/ in a menu item. The only way I can think of is to subclass MenuItem again and add the same functionality like for the title attr, to let users link to /person/{{request.user.id}}/edit, which is generated in a callable.