fix: better defined & documented .env and ENV_VAR support

[Pinjasaur]

This closes #13.

Briefly: from the beginning I had defined a .env inside the repo which got sourced along with any other supplied .env file at generation time. This was incredibly convenient, especially when building and testing locally. However, I never really liked that for the SITE_* variables, which were more or less required for the Full Opinionated Experience™, there was the potential to just have the mostly-useless ones hanging out.

And more frustratingly, because the .env was always sourced it would override anything defined at runtime e.g. SITE_URL=http://domain.tld bic .

This work fixes all that up by leaving an effectively-empty repo .env and having any defaults defined ${VAR:-default}-style in bic itself and simply printing a warning message if the important SITE_* aren't defined. Further, if someone wanted to define a variable in their .env and have it be overridable at runtime this can be done with

VAR="${VAR:-default}"

My motivation to keep the repo .env around is maybe I'll want it in the future e.g. some build metadata. Alternatively, I could use the aforementioned syntax to define the optional config (BUILD_DIR, DATE_FORMAT, SALT, and TIMEZONE) but as it stands I'd rather leave those in the source code.

[mfossen]

LGTM