Add xml_node::ensure_attribute and xml_node::ensure_child

[zeux]

These are equivalent to a named lookup (attribute/child) followed by
appending the attribute/child if an existing one is not found. The
implementation is trivial but this is fairly commonplace in code that
modifies an existing DOM.

For example, updating an existing configuration node can now be done as:

pugi::xml_node font = config.ensure_child("font");
font.ensure_attribute("size").set_value(14);

... whereas previously this would require creating new nodes/attributes explicitly:

pugi::xml_node font = config.child("font");
if (!font) font = config.append_child("font");

pugi::xml_attribute size = font.attribute("size");
if (!size) size = font.append_attribute("size");

size.set_value(14);

Fixes #409.