Allow SSH protocol links

[watsonian]

It'd be handy if you could also use SSH protocol links like [test server](ssh://[email protected]). Is there any chance of adding that to the protocol whitelist in SanitizationFilter? I don't think there should be any security implications, but I may be missing something.

[jch]

This request is similar to an earlier one about adding irc links (#39). I'm still open to PRs that parameterize the whitelist in SanitizationFilter, but for the same reasons as the other issue, we won't be adding ssh links to github.com.