Add .riot extension

`.riot` is an [official Github extension](github-linguist/linguist#4653) and it might be worth to support it. `.tag` is no longer used for Riot.js components, it can be left here in the list for backward compatibility

src/settings.js

@@ -12,6 +12,7 @@ const defaultHTMLExtensions = [
   ".nunjucks",
   ".php",
   ".tag",
+  ".riot",
   ".twig",
   ".we",
 ]