Add info methods for backend functionality

[tbreloff]

It would be nice to provide:

• warnings/errors when trying to use features that are unsupported for a given backend
• Lists of features that a backend supports
• Lists of backends that support a given feature
• Lists of backends that fully or partially support a given plot command

It would be nice for a user to try to do something, and be well directed to a different backend if they want to do something that the backend doesn't support.