GFM autolink does not take ftp URLs

[gseastream]

This is a conformance issue, although I don't really mind either way I just figured I'd bring it up. It seems that this package's autolink extension supports the FTP protocol, and even says it does so because GitHub does:

https://github.com/Zopieux/py-gfm/blob/1496ca5a752c1c8389641809331e74fbbc6aceaf/gfm/autolink.py#L18

But I just checked and GFM does not seem to be converting FTP links, and I can't find any mention of it in the GFM spec. Example:

ftp://ftp.funet.fi/pub/standards/RFC/rfc959.txt

I don't even know much about FTP links nor do I ever use them, this is just something I noticed while looking at this package. In case you wanted to either document or remove this discrepancy.