Add tstring_view, a string reference type

tstring_view is a string reference type that provides a view into a
string that is owned elsewhere (e.g., by a std::string object).

tstring_view implements the same interface as std::base_string_view in
the standard library.  When litehtml moves to C++17 consider replacing
the tstring_view implementation with the standard library
implementations (e.g., via a using statement).