library path

Hi,
I need to set the lib path for probe in my project, couse I need to embed that.
The bad of my changes here, is that is not guaranteed the back compatibility. 
You can't do anymore just:

```js
require('node-ffprobe');
```

but you need:

```js
require('node-ffprobe')();
```

I don't fully like it but I can't do better... if you have a better solution, it is welcome.