syncany formula

See https://github.com/syncany/syncany and https://www.syncany.org

Some fiddling with their install was required, but not much.

I don't really use this so I may not be the best source of information, I just wanted to mess around with it and creating a formula was preferable to using their install.

I'll reference this PR from the syncany repo and see if they have any comments.
The `test do` block runs in a temp folder by default so that sounds good.