Add support for dotenv, if present

[benbalter]

To allow folks to keep tokens and environmental variables in a site-specific .env file.

[parkr]

💗

[rriemann]

is this functionally documented?

I suggest adding one phrase to the readme.

Personally, I do not understand in which case a jekyll project can now access the variables in .env.

[DirtyF]

@rriemann see #92