Make build.js configurable via environment variables

[pldimitrov]

The current setup is not very automation-friendly. The only way to drive this from CI without baking the configuration (and secrets) into the source code is to do something like:

• generate a distribution with a dummy confiig.json that contains only placeholders
• have CI replace the placeholders with values coming from environment variables
• generate a new zipfile (duplicating the logic from function zip(files))

or craft a string with the parameters you need and feed it to build.sh via stdin, but that is even hackier, I guess.

[tuomovee]

Please check out my PR #84 when you have time. Cheers!