github_flavored_markdown: Do not filter out data URI scheme images.

See discussion of potential security implications at
microcosm-cc/bluemonday#5.
The conclusion is that it should be okay, unless there are bugs in
client (browsers) code that would open new attack vectors. In the
interest of science and moving tech forward, I will stop filtering
these out. If you're dealing with untrusted user generated input and
feel paranoid, you may want to apply a stricter sanitization policy
that applies to your specific scenario on top.