value of presence statement #36

In YANG container nodes can have presence statement that accepts string as value (https://www.rfc-editor.org/rfc/rfc7950.html#section-7.5.5)

Currently libyang-cpp does not expose any method to read this string. The only thing present is the Container::isPresence() method that returns boolean:

It would be nice if there would be a Container::presence() method that returns the presence statement value as std:string.

Would it be OK to add it? I can make a PR.