Add new scriptlet — 'trusted-add-class'

The `trusted-add-class` would provide an opportunity to add any class name to an element. For example, sometimes it happens that a script checks the body class list.
```
example.com#%#//scriptlet('trusted-add-class', selector, classNames[, applying])
```
where
- `classNames` can be a single class or multiple classes separated by commas, e.g. `foo` or `foo,bar`
- `applying` can be `asap`, `complete` and `stay` like in [remove-attr](https://github.com/AdguardTeam/Scriptlets/blob/master/wiki/about-scriptlets.md#-%EF%B8%8F-remove-attr)

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Can be used on:
* myonlineradio.hu - bypass antiadblock.