Support for multiple XML tags with the same name

In `custom_elements`, it would be nice to have support for multiple instances of the same tag. EG: [`dc:creator` allows for multiple instances of it in the XML (as does various items)](http://www.rssboard.org/rss-profile#namespace-elements-dublin-creator):

```
Person 1
Person 2
```

But upon using an array to do so:

```
custom_elements: [
{"dc:creator": authors.map(author => author.name) },
]
```

It only lists a single `dc:creator` XML element