Add new plugin to autocomplete fabric commands

plugins/fabric/fabric.plugin.zsh

+#compdef fab
+
+_fab_list() {
+    reply=(`fab --shortlist`)
+}
+compctl -K _fab_list fab
+
+# DECLARION: This plugin was created by kennethreitz. What I did is just making a portal from https://github.com/kennethreitz-archive/fabric-zsh-completion.