Load config from user config directory

[Doridian]

This does, despite how it looks, not change default behavior.

This addresses two concerns:

• The CLI parameter's default being set to DefaultConfigName means the default logic was never run
• The default logic had two drawbacks
  • It would actually run the *cfg = ... path on success, making it effectively useless
  • It could only load one file

It addresses this as follows:

• Make the default config file name the empty string, allowing the defaulting logic to hit
• Add a function which can take an arbitrary amount of config file names and yields the results of the first one that actually exists

All the go.mod and go.sum changes are from go mod tidy