This change follows this proposed behaviour:
Ctrl+Shift+Left: move to last visited directory
Ctrl+Shift+Right: move to next visited directory
an alternative behaviour would be:
Ctrl+Shift+Left: move directory to the left in `dirs` output
Ctrl+Shift+Right: move directory to the right in `dirs` output
It also introduces `setopt nopushdminus` as a way to standardise
pushd syntax. It's value wasn't clear before, which has been the
cause of so many pull requests regarding this plugin not working
in some environments.
Marc Cornellà
Rotem Yaari