Add environment defaults and add docs

[ropottnik]

closes #4

@Jeremy-Stafford if you decide to merge this PR, could you then also make a new release on pypi? Thanks.

[ropottnik]

@Jeremy-Stafford could you have a look?

[Jeremy-Stafford]

Sorry for the rather slow response, GitHub was supposed to send me an email notification, but did not.

I would suggest the following changes:

• Improve formatting of doc-strings: there should be a summary of each function on the line with the """, then a double line break, then any extra details, preferably in paragraphs.
• Put all the imports together, after the module-level docstring.
• I think has_env_default_factory and get_corresponding_env_var are unecessary and that using default_factory directly is a better way to go. Something like

import os
from typing import *

def from_env_var(name: str) -> Function[str]:
     return lambda: os.getenv(name)

Perhaps with some extra attributes for the lambda.

I currently don't have access to a computer (for about 1 week), so I can't write code very well or update the package on PyPI.