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.

plugins/dircycle/dircycle.plugin.zsh

@@ -1,10 +1,27 @@
-##
-# dircycle plugin: enables cycling through the directory
-# stack using Ctrl+Shift+Left/Right
+# enables cycling through the directory stack using
+# Ctrl+Shift+Left/Right
+#
+# left/right direction follows the order in which directories
+# were visited, like left/right arrows do in a browser
 
-eval "insert-cycledleft () { zle push-line; LBUFFER='pushd -q +1'; zle accept-line }"
+# NO_PUSHD_MINUS syntax:
+#  pushd +N: start counting from left of `dirs' output
+#  pushd -N: start counting from right of `dirs' output
+setopt nopushdminus
+
+insert-cycledleft () {
+	zle push-line
+	LBUFFER='pushd -q +1'
+	zle accept-line
+}
 zle -N insert-cycledleft
-bindkey "\e[1;6D" insert-cycledleft
-eval "insert-cycledright () { zle push-line; LBUFFER='pushd -q +0'; zle accept-line }"
+
+insert-cycledright () {
+	zle push-line
+	LBUFFER='pushd -q -0'
+	zle accept-line
+}
 zle -N insert-cycledright
+
+bindkey "\e[1;6D" insert-cycledleft
 bindkey "\e[1;6C" insert-cycledright