Reader merge and overlay support

[kyle-figure]

• Add Reader::merge() and Reader::applyOverlay() functions
• Support operator== and operator!= for Reader class
• Move Reader::getCfgMap() to public function. This is needed for Reader::merge() and Reader::applyOverlay()
• Add new config_merge_test.cpp test suite
• Support operator== for ConfigBase class
• Add compareNestedMaps() and mergeLeft() helpers for CfgMap class