Add custom HTML alert type support

Add new alert type: **HTML**

This will allow creating HTML alert box, can be use to create custome dialog types, for example: Prompt, Date picker,...

Usage:

~~~javascript
swal({   
    title: "Custom dialog!",   
    text: "Here's my <b>HTML</b> <u>message</u>!",   
    type: "html",   
    confirmButtonText: "Yay!" 
});
~~~