[FEATURE] Allow support for waitUntil configuration

[fabiangarga]

I implemented this PR to resolve #754 . Added the waitUntil option for the rendertron configuration.

How to use the configuration update:

(config.json)

{
    ...
    "waitUntil": [
        "domcontentloaded",
        "networkidle2"
    ]
}

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[fabiangarga]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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@googlebot I signed it!

[fabiangarga]

@googlebot I signed it!