URL link spoofing

Words such as `http://example.com` and `example.com` included in the message are displayed by URL link. This URL link naturally links to `example.com`. However, we can spoof the link destination by changing the message post request.

 POST /api/chat.postMessage HTTP/1.1
 Host: example.slack.com
 ...

 ...
 -----------------------------87462859699239992111770463
 Content-Disposition: form-data; name="text"

-http://example.com
+<http://evil.com|http://example.com>
 -----------------------------87462859699239992111770463
 ...