feat: Adds option in memoize to ignore args

[Darkheir]

This PR adds an option to ignore some arguments while generating the cache key.

i.e.

@cache.memoize(50, args_to_ignore=["b"])
def big_foo(a, b):
    return a + b + random.randrange(0, 100000)

assert big_foo(5, 3) == big_foo(5, 2)  # True

[coveralls]

Coverage increased (+0.1%) to 78.462% when pulling f082d5d on Darkheir:feat/memoize_ignore_args into 9e1b157 on sh4nks:master.

[sh4nks]

Looks good. Thanks for your contribution!