Fix: loading guard before cpo-save

If script finishes after cpo-save, &cpo remains as cpo&vim.
It is better to check loading guard before cpo-save.
Merge pull request #31 from rhysd/fix/cposave

Fix: loading guard should be before cpo-save
Use s: prefixed variable names for scope.
Instead of setting number just call nornu.

Really the use of the number option should be part of the plugin but
the current integration is poor. The nu and rnu means something different than
nonu and rnu for instance.