code: function _from_env (opts, cfg_so_far, cb) {
    var ka = [];
    var va = [];
  
    _.forEach (process.env, function (v, k) {
      if (opts.regexp && !(k.match (opts.regexp))) return;
      if (opts.prefix && !(_.startsWith (k, opts.prefix))) return;
  
      if (opts.prefix) k = k.substr (opts.prefix.length);
  
      // change __ into . in k
      k = k.replace (/__/g, '.');
  
      ka.push (k);
      va.push (v);
    });
  
    var obj = _.zipObjectDeep(ka, va);
    _expand (obj, cfg_so_far, cb);
  }
docs: /////////////////////////////////////////// gets data from env