PEGTL 1.2.0

• Added pegtl_string_t and pegtl_istring_t to simplify string definitions as follows:

   pegtl::string< 'h', 'e', 'l', 'l', 'o' >  // Normal
   pegtl_string_t( "hello" )                 // New shortcut

• Added examples/abnf2pegtl.cc application that converts grammars based on ABNF (RFC 5234) into a PEGTL C++ grammar.
• Added contrib/alphabet.hh with integer constants for alphabetic ASCII letters.