Fix bug where output from settings wasn’t honoured

If `output` is given (has a type of `boolean`) in the configuration
for `engine()`, it should overwrite configuration files.  Previously,
this didn’t work.

lib/index.js

@@ -58,7 +58,7 @@ function run(options, callback) {
   /* Output. */
   settings.streamOut = options.streamOut || process.stdout;
   settings.streamError = options.streamError || process.stderr;
-  settings.output = options.output || undefined;
+  settings.output = options.output;
   settings.out = options.out;
 
   if (settings.output && settings.out) {