Middleware to turn off caching
  ==============================
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  It's possible that you've got bugs in an old HTML or JavaScript file, and with a cache, some users will be stuck with those old versions. This will (try to) abolish all client-side caching.
  
  ```javascript
  var nocache = require('nocache')
  app.use(nocache())
  ```
  
  This sets four headers, disabling a lot of browser caching:
  
  - `Cache-Control: no-store, no-cache, must-revalidate, proxy-revalidate`
  - `Pragma: no-cache`
  - `Expires: 0`
  - `Surrogate-Control: no-store`
  
  If you want to crush the `ETag` header as well, you can:
  
  ```javascript
  app.use(nocache({ noEtag: true }))
  ```
  
  Caching has some real benefits, and you lose many of them here. Browsers won't cache resources with this enabled, although *some* performance is retained if you keep ETag support. It's also possible that you'll introduce *new* bugs and you'll wish people had old resources cached, but that's less likely.