Add "start" and "end" properties

This gives the starting (lineno, column) and ending (lineno, column) for
a logical line.
diff --git a/yapf/yapflib/unwrapped_line.py b/yapf/yapflib/unwrapped_line.py
index 778cd8e..3b480a0 100644
--- a/yapf/yapflib/unwrapped_line.py
+++ b/yapf/yapflib/unwrapped_line.py
@@ -212,6 +212,24 @@
     return self.first.lineno
 
   @property
+  def start(self):
+    """The start of the logical line.
+
+    Returns:
+      A tuple of the starting line number and column.
+    """
+    return (self.first.lineno, self.first.column)
+
+  @property
+  def end(self):
+    """The end of the logical line.
+
+    Returns:
+      A tuple of the ending line number and column.
+    """
+    return (self.last.lineno, self.last.column + len(self.last.value))
+
+  @property
   def is_comment(self):
     return self.first.is_comment