Enable additional / alternative config via environment variables

[reece]

config.py reads the default config file from _data/defaults.ini.

We should provide a mechanism to also read a custom config file (e.g., specified by env var HGVS_CONFIG_FILE) that overrides values in defaults.ini. We should always read defaults so that we can guarantee that config options are always set.

[github-actions[bot]]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[github-actions[bot]]

This issue was closed because it has been stalled for 7 days with no activity.

[reece]

This issue was closed by stalebot. It has been reopened to give more time for community review. See biocommons coding guidelines for stale issue and pull request policies. This resurrection is expected to be a one-time event.