Remove any script tags from generated feed xml

Hey! 👋

In my opinion `node-rss` should remove all script (e.g. iframe) tags from a generated feed, because some rss validators show the following warnings/recommendations:

`content:encoded should not contain iframe tag`
`description should not contain iframe tag`

These warnings occur if you add `custom_elements` like
```
item.custom_elements.push({
'content:encoded': {
_cdata: ""
}
});
```

Refs https://validator.w3.org/feed/docs/warning/SecurityRisk.html

As this is a rule, it might be helpful for everybody. Let me know if you think that the caller should remove script tags by himself.