PEGTL 2.2.0

• Added possibility for actions' apply() or apply0()-methods to return a bool which is then used to determine overall success or failure of the rule to which such an action was attached.
• Added <tao/pegtl/contrib/parse_tree.hpp> and the examples/parse_tree.cpp application that shows how to build a parse tree. The example goes beyond a traditional parse tree and demonstrates how to select which nodes to include in the parse tree and how to transform the nodes into an AST-like structure.
• Added bom rules for UTF-8, UTF-16 and UTF-32.
• Added some missing includes for config.hpp.
• Added automated testing with Clang 5.
• Added automated testing with Xcode 9.