add 'look_for_alphanumeric_id' flag to enable parsing values that are not only digits

[JonasDoesThings]

This PR adds the settings flag "look_for_alphanumeric_id" which can be set to true to allow for parsing non-numeric IDs. It is false by default, so nothing changes with existing configs. I've also created testcases for this feature.

Example:

- reference_prefix: GIT@
  target_url: 'https://github.com/<num>'
  look_for_alphanumeric_id: true

transforms: GIT@k4ds3/plavatar into https://github.com/k4ds3/plavatar

Feel free to merge this if you find it useful too.