Add a rot13 cipher in response to Issue RisingLight#7

Author: godfreyduke

Ciphers/rot13.py

@@ -0,0 +1,21 @@
+#rot13
+OFFSET_LOWER = ord('a')
+OFFSET_UPPER = ord('A')
+
+def rot13(text):
+  rot13_text = ''
+  for i in range(len(text)):
+    ch = text[i]
+    if ch.isalpha():
+      if(ch.islower()):
+        offset = OFFSET_LOWER
+      else:
+        offset = OFFSET_UPPER
+      rot13_text += chr(((ord(ch)-offset+13)%26+offset))
+    else:
+      rot13_text += ch
+  return rot13_text
+
+plain_text = input("Enter the string: ")
+print(rot13(plain_text))
+print(rot13(rot13(plain_text)))