Add support for cygwin open in web-search plugin

plugins/web-search/web-search.plugin.zsh

@@ -16,6 +16,7 @@ function web_search() {
   # define the open command
   case "$OSTYPE" in
     darwin*)  open_cmd="open" ;;
+    cygwin*)  open_cmd="cygstart" ;;
     linux*)   open_cmd="xdg-open" ;;
     *)        echo "Platform $OSTYPE not supported"
               return 1