All `feedOptions` and `itemOptions` should be camel case

There are some options that are snake case:
* feed_url
* site_url
* image_url
* custom_namespaces
* custom_elements

And there are others that are camel case:
* managingEditor
* webMaster
* pubDate

I think it would be nice if all snake case options were camel case to keep it more consistent. You don't have to break backwards compatibility for that, you can keep the snake case ones as deprecated aliases to their camel case alternatives and change them in the docs.