Read R lists

[ofajardo]

It would be nice if librdata could read R lists.

It has been mentioned before in #12 , but as more and more people is asking about it (for example: this and this) I create here a separate issue for more visibility.

[ofajardo]

In the meantime it would be useful if a warning was emitted when a list is encountered in the file, many people is confused by trying to read lists and not getting anything back.