expose ast transformer - fixes #263

doc/usage.md

@@ -95,6 +95,15 @@ to5.transformFile("filename.js", options, function (err, result) {
 });
 ```
 
+### to5.transform.fromAst(ast, [code], [opts])
+
+```javascript
+var result = to5.transform(ast, "var a = 2;", opts);
+result.code;
+result.map;
+result.ast;
+```
+
 #### Options
 
 ```javascript