title: Add a preloader to rmarkdown HTML output [flexdashboard]
body: <p>I have a relatively complex <code>flexdashboard</code> which takes some time to load.</p>
  
  <p>Does anyone have experience with adding a preloader to <code>flex_dashboard</code> output?</p>
  
  <p>For instance, adding any of <a href="https://medium.muz.li/top-30-most-captivating-preloaders-for-your-website-95ed1beff99d" rel="nofollow noreferrer">these</a> would be great for me. While I know how to add a <code>gif</code>, I'm wondering how to stop displaying it at website load.</p>
  
  <p>Is anything like that possible within <code>rmarkdown</code> as well as <code>flex_dashboard</code>?</p>