Add filter "getArticleByPath" for theme templates

lib/output/website.js

@@ -96,6 +96,14 @@ WebsiteOutput.prototype.prepare = function() {
             return that.toURL(s);
         });
 
+        // Get an article using its path
+        that.env.addFilter('getArticleByPath', function(s) {
+            var article = that.book.summary.getArticle(s);
+            if (!article) return undefined;
+
+            return article.getContext();
+        });
+
         // Relase path to an asset
         that.env.addFilter('resolveAsset', function(href) {
             href = path.join('gitbook', href);