Script to upload CRX file shouldn't require version number parameter

[emerick]

When we package a CRX file, we require the user to specify a version number via --set-version. This makes sense, as the manifest.json requires a version number.

However, our upload script also requires the --set-version parameter. This seems redundant and error-prone (it's easy to make a typo and specify a version number that doesn't match the version number in the CRX's manifest.json). Instead of requiring the user to pass --set-version when uploading, we should just unzip the CRX to a temporary directory and parse the version number out of its manifest.json.