function command(settings) {
  var options = settings.execOptions;
  var executable = options.exec;
  var args = [];

  // after "executable" go the exec args (like --debug, etc)
  if (options.execArgs) {
    [].push.apply(args, options.execArgs);
  }

  // then goes the user's script arguments
  if (options.args) {
    [].push.apply(args, options.args);
  }

  // after the "executable" goes the user's script
  if (options.script) {
    args.splice((options.scriptPosition || 0) +
      options.execArgs.length, 0, options.script);
  }

  return {
    executable: executable,
    args: args,
  };
}
command constructs the executable command to run in a shell including the
user script, the command arguments.

@param  {Object} settings Object as:
                          { execOptions: {
                              exec: String,
                              [script: String],
                              [scriptPosition: Number],
                              [execArgs: Array<string>]
                            }
                          }
@return {Object}          an object with the node executable and the
                          arguments to the command