Allow opening other files, not just .env

[nvtkaszpir]

Currently environs read only hard-coded .env files, and only path is available.
Yet more and more tools allow custom .env files.

I believe environs should add parameter like file='.env' to be able to override defaults.

[c-w]

Note that it's already possible to read files not called .env when using recurse=False, so perhaps that offers a work-around:

from environs import Env

with open("test.txt", "w") as fobj:
    fobj.write("A=foo\n")
    fobj.write("B=123\n")

env = Env()
env.read_env("test.txt", recurse=False)

assert env("A") == "foo"
assert env.int("B") == 123

[nvtkaszpir]

Oh right, that's PEBKAC from my point. I was totally not seeing part of the code.

I believe this should be added to the docs as an example.

[sloria]

This is now documented here: https://github.com/sloria/environs#reading-a-specific-file