add option callback on init

This change allows the developer to add a callback function on init event.


To use it, just add in your `config.yml` file :

    stfalcon_tinymce:
        init: true

Next, you just have to add a function named `callback_tinymce_init()` in your javascript like this :

    function callback_tinymce_init(){
        // script called on tinymce initialization ...
    }