Clean up and fix +1/-0 syntax to work as expected
Clean up and fix +1/-0 syntax to work as expected

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.

FrankSansC/oh-my-zsh,nunogt/oh-my-zsh,FrankSansC/oh-my-zsh,lightmanhk/oh-my-zsh,nunogt/oh-my-zsh,tekniklr/oh-my-zsh,FrankSansC/oh-my-zsh,lightmanhk/oh-my-zsh,lightmanhk/oh-my-zsh,FrankSansC/oh-my-zsh,tekniklr/oh-my-zsh,lightmanhk/oh-my-zsh