Breaking change: add a BaseFile ABC of which File is a subclass. Root nodes can now be all types of JSON values, not just objects.
Add SFTPFile class which uses paramiko to represent remote files
Document new, backwards-incompatible API in readme

Also add an SFTPFile example to the Usage section.
Fix insert_value_at_key_path and add PythonFile
Fix interactive Python session in usage example
Add MultiFile with preliminary writing behavior
Document MultiFile and PythonFile
Add automatic version parsing
Version bump