Use default reporter when none specified in config

This addresses [Issue#369], where using a config file that doesn't
specify a reporter breaks. We update the code so that when there's a
config without a reporter, we fallback to the default one.

[issue#369]: tcort#369

markdown-link-check

@@ -288,7 +288,7 @@ async function processInput(filenameForOutput, stream, opts) {
         opts.retryCount = config.retryCount;
         opts.fallbackRetryDelay = config.fallbackRetryDelay;
         opts.aliveStatusCodes = config.aliveStatusCodes;
-        opts.reporters = config.reporters;
+        opts.reporters = config.reporters ?? opts.reporters;
     }
 
     await runMarkdownLinkCheck(filenameForOutput, markdown, opts);