Support CRX3

[mike10004]

They went and came out with a new CRX format without telling me: https://bugs.chromium.org/p/chromium/issues/detail?id=720092

Support must be added.

p.s. I didn't even know what we had been supporting was CRX2, if indeed that's the case; is there a CRX1 we should also be worried about?

[qclucky7]

When an extension file is unpacked with multiple public keys, the first one is not the extension's public key。
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