Proposed API: this.iWillCallOldFn() for extendFunction NOT to call oldFn 

First draft of docs:
By default, if you don't call the oldFunction, the function you're extending, then extendFunction will call it for you, and return it's return value.

If you plan on calling the oldFunction asynchronously and need to tell extendFunction.js NOT to call the oldFunction itself, simply call:
```javascript
this.iWillCallOldFn();
```
In your extension (`extendFunction('alert', function alertExtension() { this.iWillCallOldFn(); ... `

Shorter write up:
If you want to call the oldFunction you're overriding asynchrously, you need to tell extendFunction that. By default, extendFunction calls the oldFunction and returns it's return value