Ability to specify dataset attributes

Sometimes it is nice/necessary to specific dataset attributes instead of redundantly sticking them in the properties of each feature, such as summary statistics like min and max. For an example, please see changes to README.md.

Ideally, these properties would be placed before "features" for readability, but it seemed like there would be a few major changes that would need to take place to accomplish that, and I'm not sure it's worth it given that GeoJSON files are usually read programmatically.

Just a small change that I found beneficial and thought I would share, but I understand if you don't want to bog it down with additional features. Let me know what you think!