Use a binary comprehension

Instead of converting to a charlist and back.

README.md

@@ -95,3 +95,9 @@ LineBot.APIClient.get("richmenu/list")
 ### Flex Message Update
 
 Recent additions to the Flex message API have not been added yet.
+
+## Online Documentation
+
+Documentation for this package is available online at [Hexdocs](https://hexdocs.pm/line_bot/) and interactively in iex via [`h/1`](https://hexdocs.pm/iex/IEx.Helpers.html#h/1), for example:
+
+    iex> h LineBot

sample/lib/line_bot_sample.ex

@@ -182,9 +182,7 @@ defmodule LineBotSample do
       code
       |> String.trim()
       |> String.upcase()
-      |> String.to_charlist()
-      |> Enum.map(&(&1 + 0x1F1A5))
-      |> List.to_string()
+      |> (&for(<<cp::utf8 <- &1>>, into: "", do: <<cp + 0x1F1A5::utf8>>)).()
 
     log_and_reply(request_message, %Message.Text{text: code}, reply_token)
   end