HTTP Public Key Pinning (HPKP) middleware
  =========================================
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  Adds Public Key Pinning headers to Express/Connect applications. To learn more about HPKP, check out [the spec](https://tools.ietf.org/html/rfc7469), [the article on MDN](https://developer.mozilla.org/en-US/docs/Web/Security/Public_Key_Pinning), and [this tutorial](https://timtaubert.de/blog/2014/10/http-public-key-pinning-explained/).
  
  Usage:
  
  ```js
  var express = require('express')
  var hpkp = require('hpkp')
  
  var app = express()
  
  var ninetyDaysInMilliseconds = 7776000000
  app.use(hpkp({
    maxAge: ninetyDaysInMilliseconds,
    sha256s: ['AbCdEf123=', 'ZyXwVu456='],
    includeSubdomains: true,         // optional
    reportUri: 'http://example.com', // optional
    reportOnly: false,               // optional
  
    // Set the header based on a condition.
    // This is optional.
    setIf: function (req, res) {
      return req.secure
    }
  }))
  ```
  
  Setting `reportOnly` to `true` will change the header from `Public-Key-Pins` to `Public-Key-Pins-Report-Only`.