Bind self._safe_protocols to extras

[Wtower]

In:

https://github.com/trentm/python-markdown2/blob/master/lib/markdown2.py#L1274

_safe_protocols = re.compile(r'(https?|ftp):', re.I)

I would recommend that this is not hard-coded but rather there is an extra for it.

Alternatively, I would recommend to allow additional URL schemes such as mailto and tel.