Add segment

[yields]

Add Segment, closes #3418.

<amp-analytics type="segment">
<script type="application/json">
  {
    "vars": {
      "writeKey": "<key>",
    }
  }
...

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[cramforce]

We automatically generate a golden file test for this that you will need to update. See https://travis-ci.org/ampproject/amphtml/builds/138287933

[avimehta]

lgtm. Thanks for adding the support!

[jridgewell]

@yields: Have you signed the CLA yet?

[calvinfo]

We're just double checking with the lawyer that it's all good, but we should be ready to go shortly! :)

[googlebot]

CLAs look good, thanks!

[yields]

@jridgewell signed and re-based!