Support SSH downloads

[ficd0]

Usually, if someone has a private repo, they also have SSH access configured for it. Therefore, I think we could make the ability to gitsnip from private repos provider-agnostic (not exclusive to GitHub via API token). Instead, perhaps if the user provides an SSH link for the repo URL, gitsnip can try to clone via SSH -- which will work if the user has the proper keys already set up in their SSH config.

# downloading from private repo with SSH
gitsnip git@github.com:foobar shazbot/file.txt

[dagimg-dot]

I will look into this one.