Consider removing fallback value for `site_url`?

I just spent a while trying to figure out why a gatsby generated rss feed included links to this repo. After much searching, I discovered this fallback code: https://github.com/dylang/node-rss/blob/1.2.2/lib/index.js#L36. This doesn't feel like a very sane default to me. (I don't think there *is* a sane default one could have here.)

The docs in `readme.md` (https://github.com/dylang/node-rss/tree/1.2.2#feedoptions) do not declare this as optional:

```
* `site_url` **url string** Url to the site that the feed is for.
* `image_url` _optional_ **url string** Small image for feed readers to use.
```

Could we instead throw an exception if `site_url` is not specified?