Add a transform/validator that checks for files on a default path.

[phlptp]

This is an alternative to #684 and closes #682. It add a transform/validator that checks for a file and on a default path.
This if used with a config ptr can be used to add a default path, or any number of default paths for that matter.

I think it has the same capabilities as #684 without adding any additional API calls and adding a new validator which may have other uses as well.

[phlptp]

This will need a couple more test cases and some docs before merging but wanted to see what @henryiii and Firefly35 thought of this alternative.

[henryiii]

This looks good, works within the existing infrastructure, and can be expanded to look in several locations (like ~/.config and ./.config, etc). I think a bit of docs showing how to use it would be useful.

[sg-james]

Apologies for bumping, but I believe this PR might have accidentally closed issue #694 by accident and meant to close issue #682.

[henryiii]

Corrected, thanks!