add reference_border_num option

Author: t-yamamura

Diff for: border.py

@@ -0,0 +1,13 @@
+class Border(object):
+    def __init__(self, before, after, score, cand):
+        """
+
+        :param int before:
+        :param int after:
+        :param float score:
+        :param bool cand:
+        """
+        self.before = before
+        self.after = after
+        self.score = score
+        self.cand = cand