Allow pretty printing XML in unparse()

[martinblech]

xmltodict should have the option to pretty print XML with correct indentation. The default behavior should stay the same (print all inline), and this new feature should only be enabled with an optional argument to unparse().

[martinblech]

I think the right, robust way to do this is by leaving XMLGenerator untouched and modifying our own _emit function to pass a depth parameter to itself and call content_handler.ignorableWhitespace() appropriately.