title: Hashed file names using Rollup
body: <p>I am fairly new to bundlers and <code>rollup</code> specifically.  What is the best way to conditionally hash file names in <code>rollup.config.js</code> when building for production,  while at the same time saving the <code>index.html</code> to reference the new <code>.css</code> and <code>.js</code>  hashed versions? </p>
  
  <p>I see this in the <a href="https://rollupjs.org/guide/en/#outputentryfilenames" rel="noreferrer">docs</a>, but I guess I don't know how to conditionally set these options based on <code>dev/prod</code> settings? </p>
  
  <pre><code>output: {
          sourcemap: true,
          format: 'iife',
          name: 'app',
          file: 'public/build/bundle.js'
         // entryFileNames : 'bundle[hash].js
      },
  </code></pre>
  
  <p>or is using <code>rollup-plugin-hash</code> a better solution? 
  still not sure what the best practice would be for updating the <code>index.html</code> 
  (and what does the manifest file option provide?) </p>