Create YamlFileReader from String (and not only from file)

[ljacqu]

Offer a functionality to create a YamlFileReader from a String rather than a File/Path. Interesting for testing or for other usages where the YAML comes from a source other than a file. Probably means removing the Path path field from YamlFileReader, and then the name of YAML file reader would no longer be fully accurate? So maybe better to put this on the back burner or to only do it for ConfigMe 2.0 so we could potentially rename the class as well