Support environment variables and .env file

[josephabrahams]

Since many projects/devs are used to using .env files to store secrets, and tools like Foreman read these, it would be nice if gokusan followed this convention.

Ideal behavior:
Gokusan looks for .env file in current directory. If it finds it, it reads in those variables into the current environment. Gokusan uses environment variables throughout the code so that the project will fall back to environment variables that are passed in as well.

[jcarbaugh]

It would also be great to look for a ~/.gokusan file first, then update with .env or other locally defined config files. That way someone could have their main credentials stored in the user file, with bucket info in the project.