Add --read-only to nsync

This prevents nsync from even attempting to push anything to a git
repo.
Add --parallel N to nsync

This sets the parallelism to N when running nsync, as opposed to the
default of 8.
Add .nsyncrc.local to nsync

This allows me to have a machine-specific configuration file for
nsync, which overrides the global config file (which I store in git).
Add SKIP command to nsyncrc

This allows me to set a particular repo to be skipped by nsync, which
is probably most useful when used in conjunction with .nsyncrc.local.