Add separator to Dock

- Add a spacer to the application (left) side of the dock:

defaults write com.apple.dock persistent-apps -array-add '{tile-data={}; tile-type="spacer-tile";}'

- Add a spacer to the other (right) side of the dock:

defaults write com.apple.dock persistent-others -array-add '{tile-data={}; tile-type="spacer-tile";}'

Then, use 'killall Dock' to restart the Dock.