add watermark, test=doc (#2596)

tools/watermark.py

+# add watermark for text
+def watermark(content, pattern):
+    m = list(zip(pattern * (len(content) // len(pattern) + 1), content))
+    return ''.join([x for t in m
+                    for x in t] + [pattern[len(content) % len(pattern)]])
+
+
+# remove cyclic watermark in text
+def iwatermark(content):
+    e = [x for i, x in enumerate(content) if i % 2 == 0]
+    o = [x for i, x in enumerate(content) if i % 2 != 0]
+    for i in range(1, len(e) // 2 + 1):
+        if e[i:] == e[:-i]:
+            return ''.join(o)
+    return ''.join(e)
+
+
+if __name__ == "__main__":
+    print(watermark('跟世龙对齐 Triton 开发计划', 'hbzs'))
+    print(iwatermark('h跟b世z龙s对h齐b zTsrhibtzosnh b开z发s计h划b'))