Allow `--sync-level=size` in transfer submissions
Allow `--sync-level=size` in transfer submissions

Adds this sync level setting, which is passed through to the SDK as-is.

Fixes #66.

Requires globus/globus-sdk-python#66 to work end-to-end, but won't break
anything if that isn't in-place.

globus/globus-cli,globus/globus-cli