Add support non-zero-terminated strings (pointer+size, i.e. std::string_view)

[Hedede]

I use std::string_view extensively in my code, because it allows to make substrings very cheaply (requires only adjusting a couple of pointers instead of copying the string), but pugixml accepts only const char* arguments, which forces me to make local copies of strings.

It would nice to have either a std::string_view overload, or const char*, size_t overload.

[rettinghaus]

See #73