Accept dictionary default value for env.dict

[c-w]

Currently, env.list accepts a list default value, so the following code works:

>>> my_list = env.list("LIST", ["1", "2"])

However, env.dict does not accept a dict default value, so the following code crashes:

>>> my_dict = env.dict("DICT", {"key1": "1", "key2": "2"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../site-packages/environs.py", line 54, in method
    value = preprocess(value, subcast=subcast, **kwargs)
  File ".../site-packages/environs.py", line 114, in _preprocess_dict
    for key, val in (item.split("=") for item in value.split(",") if value)
AttributeError: 'dict' object has no attribute 'split'

This lack of similarity between the APIs is somewhat confusing, so this pull request proposes a change that makes env.dict accept a dictionary as default argument.

[sloria]

Thanks!