Update gitignore.plugin: comma separated arguments

Seems like this is more like the intended effect, splits arguments with commas
for proper gitignore.io api request.
Thanks for teaching me about custom compdef completions!

Updated with zsh join operator by Marc Cornellà

plugins/gitignore/gitignore.plugin.zsh

-function gi() { curl -sL https://www.gitignore.io/api/$@ ;}
+function gi() { curl -sL https://www.gitignore.io/api/${(j:,:)@} }
 
 _gitignoreio_get_command_list() {
   curl -sL https://www.gitignore.io/api/list | tr "," "\n"