Removed volume name as an arg to sync:start

[febbraro]

An example of what would be needed for #85

[grayside]

This makes sense to me. It means the only edge case on finding the sync volume for a project is whether the project uses the --dir and does not configure a volume name. That seems like something for #58/documentation to incorporate as best practice guidance.

#85 includes the more outrageous proposal of taking the idea of #6 to have a rig data or rig fs command, and include ad hoc unison sync volume management as a sub-component there and require rig project sync to be explicitly configured. I think that can wait for use cases/v3.