Support a dict-like variant to define validation functions

[smarie]

this could be cool

validate('x', x, custom={'x should be finite': (isfinite, NotFinite),
                                 'x should be even': is_even,
                                 'x should be strictly positive': (i > 0)})

or

validate('x', x, custom={isfinite: ('x should be finite', NotFinite),
                                 is_even: 'x should be even',
                                 (i > 0): 'x should be strictly positive'})

[smarie]

Original idea came from field() in mixture: https://github.com/smarie/python-mixture/issues/2

[smarie]

Fixed in 5b8c009