Use $OSTYPE instead of uname to speed things up

The $OSTYPE variable is set at ZSH compile time and can be safely used
to determine the OS of the system. e.g. darwin (os x)

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

@@ -4,7 +4,7 @@ function web_search() {
 
   # get the open command
   local open_cmd
-  if [[ $(uname -s) == 'Darwin' ]]; then
+  if [[ "$OSTYPE" = darwin* ]]; then
     open_cmd='open'
   else
     open_cmd='xdg-open'