Refactore xml config parser in order to improve exensibility

.gitignore

Api/ConfigParserInterface.php

Model/Config/Converter.php

Model/Config/Converter/BlockNodesParser.php

Model/Config/Converter/ContentHeadingParser.php

Model/Config/Converter/PageNodesParser.php

Model/Config/Converter/TitleNodeParser.php

Model/Config/NodeConverter.php

Model/Config/Parser/ContentHeadingParser.php

Model/Config/Parser/ContentParser.php

Model/Config/Parser/CustomDesignParser.php

Model/Config/Parser/DesignParser.php

Model/Config/Parser/MetaDataParser.php

Model/Config/Parser/ParserChain.php

Model/Config/Parser/Query/FetchAttributeValue.php

Model/Config/Parser/Query/FetchBooleanAttributeValue.php

Model/Config/Parser/Query/FetchChildNodeValue.php

Model/Config/Parser/SeoParser.php

Model/Config/Parser/StoresParser.php

etc/di.xml