Support gtag consent mode in gatsby-plugin-google-gtag

[chrishilditch]

It'd be nice if the gatsby-plugin-google-gtag plugin supported gtag's consent option.

From the google docs - 'Consent mode (beta) allows you to adjust how your Google tags behave based on the consent status of your users. You can indicate whether consent has been granted for Analytics and Ads cookies. Google's tags will dynamically adapt, only utilizing measurement tools for the specified purposes when consent has been given by the user.'

The plugin current allows you to set other options.

I think it'd be nice if you could set the default consent option in gatsby-config.js, along with the other gtag options. And add a JS example to the documention to show how to update the consent settings.

I can put together a PR for this.