Add method to iterate over passed XML

This is purely sugar, but it stops me from having to create a temp file or invoke io.StringIO myself. We already import io, so I don't think it adds any real overhead.

Author: Protonk

mw/xml_dump/iteration/iterator.py

@@ -186,6 +186,13 @@ def from_file(cls, f):
         assert element.tag == "mediawiki"
         return cls.from_element(element)
 
+    @classmethod
+    def from_ref(cls, ref):
+        f = io.StringIO(ref)
+        element = ElementIterator.from_file(f)
+        assert element.tag == "mediawiki"
+        return cls.from_element(element)
+    
     @classmethod
     def from_page_xml(cls, page_xml):
         header = """