Decision: Allow invalid config settings, automatically use default setting in their place

[atc0005]

While working on #161 I dealt with a case (pun intended) where I forgot to deal with a "stop the world" error and the code path descended into a switch statement which assumed only valid values would be provided.

This could be worked around in multiple ways, but these appear to be the most "accessible" for my current skill level:

• panic immediately upon finding an invalid value, even if it would otherwise be overridden later by another config source
  • this might be tricky for the current state of the code
• panic at the end after merging all configuration sources (file, env vars, flags) and finding invalid values
• emit warnings, then replace invalid values with valid default values

For now, I'll plan to implement early failure/panic when finding invalid values after merging all config sources. This can either be the final behavior or can be enhanced by early failure upon first encountering an invalid value.

[atc0005]

• emit warnings, then replace invalid values with valid default values

I should use the principle of least surprise. Having an application assume what I wanted to say when I didn't say it is a dangerous default, especially for an application in charge of pruning files. This is a no-go.