Add a basic SGML parser

I tried BeautifulSoup, which was okay, but was missing an understanding
of how OFX does SGML. That's fine, writing my own parser was not that
big of a deal

tests/test_sgml.py

@@ -0,0 +1,17 @@
+import vanth.sgml
+
+
+def child_values(node):
+    return [(child.name, child.value) for child in node.children]
+
+def test_siblings():
+    result = vanth.sgml.parse("<A><B><C>1<D>2<E>3</B></A>")
+    assert result.name == 'A'
+    assert child_values(result['B']) == [('C', '1'), ('D', '2'), ('E', '3')]
+
+def test_closing():
+    result = vanth.sgml.parse("<A><B><C>1</B><D><E>2</D></A>")
+    assert result.name == 'A'
+    assert child_values(result) == [('B', ''), ('D', '')]
+    assert child_values(result['B']) == [('C', '1')]
+    assert child_values(result['D']) == [('E', '2')]