feat: add javascript code block support (#13)

docs/examples/with-javascript.md

@@ -0,0 +1,17 @@
+```javascript run
+input.test = "Hello World";
+```
+
+::input[test]
+
+```http json
+POST https://httpbin.org/post
+
+{"input": "{{input.test}}"}
+
+```
+
+```javascript run,hidden
+// Use chai's `expect`, `assert` or `should` to make assumptions
+expect(response.body.json.input).to.equal("Hello World");
+```