Introduce optional configuration sources

[fabriziocucci]

It would be nice to specify optional sources so that the configuration build does not fail for them, e.g.

JsonNode configuration = ConfigurationBuilder.newBuilder()
    .optionalSource().fileFromPath("i-dont-exist.yaml")
    .optionalSource(new SourceNotAvailable())
    .build(JsonNode.class);

This should not fail and instead return an empty configuration if both sources are not available.