New CLI interface

usage: xapi-to-json [-h] [--version] [-v] [-o DEST] XAPIDB

CLI util to dump an xml blob of the xapi database to json

positional arguments:
  XAPIDB                Path to the xml dump of the xapi database

optional arguments:
  -h, --help            show this help message and exit
  --version             show program's version number and exit
  -v, --print-db-version
                        Include the version metadata of the extracted xapi db
                        in the '_version' key
  -o DEST, --output DEST
                        Path to the output json file. Print to stdout when
                        missing