Consider removing fallback value for site_url?

[jfly]

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?