Support AppVeyor

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Do you want to request a feature or report a bug?

Feature.

What is the current behavior?

bundlesize isn't able to recognize the secure BUNDLESIZE_GITHUB_TOKEN variable set in appveyor.yml:

WARN  github token not found
    You are missing out on some cool features.

If the current behavior is a bug, please provide the steps to reproduce.

The appveyor.yml example: https://github.com/sweetalert2/sweetalert2/blob/1c6ea81e1a6d9e80cdfb5be9ea52ede102cba4f9/.appveyor.yml

What is the expected behavior?

bundlesize is able to recognize the secure BUNDLESIZE_GITHUB_TOKEN variable set in appveyor.yml

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I know this is in the TODO in README, but it's not possible to subscribe to that, this issue is for tracking the progress on the integration with AppVeyor. Everyone who is interested can click the "Subscribe" button and be notified via GitHub notifications.