Major Features and Improvements

feature: support optional sources
feature: support empty list and single element list in flat source
feature: add config.containsRequired() (return true/false) and config.validateRequired() (throw exception instead) to check whether all values of required items exist in config or not
feature: leading or trailing whitespaces are allowed in item name / path
feature: support onSet for Item
fix: support HOCON substitutions
fix: default values in facade config now have lower priority than the values in fallback config

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