[push-api] Add authentication secret (#174)

This adds the API pieces in support of webpush-wg/webpush-encryption#4.

In short, `getKey("auth")` returns a new value that the application server is required to mix into the key derivation for encryption.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/push-api/pull/174

-- Commit Summary --

  * Adding authentication key
  * language cleanup
  * Removing duplicate ids

-- File Changes --

    M index.html (117)

-- Patch Links --

https://github.com/w3c/push-api/pull/174.patch
https://github.com/w3c/push-api/pull/174.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/pull/174