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.
