Ability to add custom quantity using shortcode

Specifying a __quantity__ when using the shortcode:
```
[add_to_cart id="1" quantity="2000"] 
```

Specifying a __custom class__ when using the shortcode:
```
[add_to_cart id="1" class="customclass"] 
```

Hiding the __price__ when using the shortcode:
```
[add_to_cart id="1" show_price="false"] 
```