Add filter fileExists

lib/output/website.js

@@ -110,6 +110,11 @@ WebsiteOutput.prototype.prepare = function() {
             return location.normalize(that.resolveForPage(this.ctx.file.path, href));
         });
 
+        // Test if a file exists
+        that.env.addFilter('fileExists', function(href) {
+            return fs.existsSync(that.resolve(href));
+        });
+
         // Transform a '.md' into a '.html' (README -> index)
         that.env.addFilter('contentURL', function(s) {
             return location.normalize(that.outputUrl(s));