Make configuration file options pluggable

[sfc-gh-mkeller]

We need to write (or find) a configuration parser that can be plugged by plugins for verification puposes. For example; what if a plugin wants to accept a file that exists and has a .txt extension.
Alternatively; we could possibly go the same way that configparser does, where verification and casting would be up to the plugins once we give them the strings, but then we'll need to add another hook, what would that look like?