⚒️ Add optional category support for alt words. (#199)

* Add optional category support for alt words.

In the frontmatter for Markdown word definitions, the `alt_words` key
now supports any number of categories, though it functionally only
supports one level in the look & feel of the website.

If we wish to use multi-level categories, we can add a class to the <ul>
in order to cleanly style nested lists.

Resolves #170

* Fix: Digital Blackface lint issues.

.eleventy.js

@@ -39,6 +39,10 @@ module.exports = function(config) {
     console.log(post);
   });
 
+  config.addFilter('isArray', function(thing) {
+    return Array.isArray(thing);
+  });
+
   config.addPlugin(pluginRss);
 
   config.addShortcode('definitionFlag', (flag) => {