Allow for hyphens in default configuration names

[forman]

Is your feature request related to a problem? Please describe.

XRLint's default configuration names are currently xrlint_config.yaml, xrlint_config.json, or xrlint_config.py.
This is quite uncommon as users would expect using hyphens in the names rather than underscores.

XRLint configurations can be passed as Python modules (default xrlint_config.py or --config <name>.py). We use dynamic imports to ingest such configurations and therefore, a Python configuration must be a valid module. Hence, the configuration's filename must be a valid Python identifier - preventing hyphens to be used.

Describe the solution you'd like

Allow at least xrlint-config.yaml and xrlint-config.json at least for the YAML and JSON formats.