Added pyrallis.set_config_type to support json and toml

Added an option to change the configuration format used by pyrallis globally, or for a specific context.

This will affect pyrallis.parse, pyrallis.dump and pyrallis.load to use the desired format

import pyrallis
pyrallis.set_config_type('json')
# or
with pyrallis.config_type('json'):
        pyrallis.dump(cfg)