DOC: now add default value in usage. Fixed #302
DOC: now add default value in usage. Fixed #302

Former-commit-id: a3a93320a41b8ca6ae37fdfd5c9df9f4e496a479 [formerly a9000713d70ac4b57bfb2f8b55f1bce1db06c564]
Former-commit-id: 145d418defb4e0a17f17cfe8fd69899f3da3b9ba
Former-commit-id: b8a673b991f3a1ab1514c9d38020b786cf638d12
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox