title: Add custom page/field to NSIS setup created with electron-builder
body: <p>I have created an Electron app which is packaged into an NSIS installer with electron-builder.</p>
  
  <p>Now I would like to add a custom text field to the installer, where the user can input a value (the value should be saved to disk/registry, it needs to be available in the app later).</p>
  
  <p>I saw there is a <code>customWelcomePage</code> macro defined in the installer, which could probably be (mis)used for this purpose? But how would I create a macro which creates a complete page? NSIS is completely new to me, and the examples on the NSIS page seem to be for standalone installers, not for hooking into an existing installer. Or is there another, better approach?</p>