title: How to filter Wordpress posts using a hook in a plugin?
body: <p>I'm creating a Wordpress plugin and, being a newbie in the development on this platform, I'm stuck on this problem.</p>
  
  <p>I'd like to have posts in the loop filtered by categories, defined by the user through an admin page. I would actually like to be able to modify query_post() parameters in the plugin, but the only trick I found is to re-run the query_post() with my user-defined criteria, thing that I would like to avoid.</p>
  
  <p>Also, due to the plugin nature, I think it make no sense to modify the theme's template.</p>
  
  <p>I'm sure the solution is evident, but can't find it! </p>