Fix block directive output contents (#42)

- add `{{contents}}` variable support to block directive output

Sources/ToucanSDK/Markdown/MarkupToHTMLVisitor.swift

@@ -317,7 +317,15 @@ struct MarkupToHTMLVisitor: MarkupVisitor {
         }
 
         if let output = block.output {
-            return output.replacingOccurrences(templateParams)
+            var contents = ""
+            for child in blockDirective.children {
+                contents += visit(child)
+            }
+
+            var params = templateParams
+            params["{{contents}}"] = contents
+
+            return output.replacingOccurrences(params)
         }
 
         if let name = block.tag {