filter_except() and map_except()

[bbayles]

This PR adds filter_except() and map_except().

filter_except() is like the built-in filter, but instead of skipping items that evaluate to False, it skips over items that raise a specific set of exceptions when validated. Here's an example using Django validators, which follow this pattern:

>>> from django.core.exceptions import ValidationError
... from django.core.validators import EmailValidator
... 
... from more_itertools import filter_except
... 
... 
... validator = EmailValidator()
... iterable = [
...     'good@example.com',
...     'good@localhost',
...     'bad',
...     'bad@bad_doman.com',
... ]
... list(filter_except(validator, iterable, ValidationError))
['good@example.com', 'good@localhost']

map_except() is like the built-in map(), but it skips over items that raise a specific set of exceptions when transformed.

>>> from django.core.exceptions import ValidationError
... from django.core.validators import EmailValidator
... 
... from more_itertools import map_except
... 
... 
... def extract_domains(address, validator=EmailValidator()):
...     validator(address)  # Raises if the address is invalid
...     return address.split('@')[1]
...     
... 
... iterable = [
...     'good@example.com',
...     'good@localhost',
...     'bad',
...     'bad@bad_doman.com',
... ]
... list(map_except(extract_domains, iterable, ValidationError))
['example.com', 'localhost']