added hub plugin from defunkt

plugins/hub/hub.plugin.zsh

@@ -0,0 +1,5 @@
+# hub alias from defunkt
+# https://github.com/defunkt/hub
+if [ $( which hub > /dev/null 2>&1 ; echo -n $? ) -lt 1 ]; then
+    eval $( hub alias -s $( ps -o comm= -p $$ ) )
+fi