Add option to not inject any CSS

[oliviertassinari]

I have to increase specificity to 4 in order to override the styles. It should be 1 or 2 at most. And ideally, not CSS would be injected in the page.

[agjohnson]

We're still deciding if we want to support ad placements without stylesheets. There are definitely some good use cases for this. However, we also need to ensure a consistent base experience for the ad placements.

For now, our immediate focus is on a consistent base experience, but we have talked about this as a future feature of the client.

Reducing the required specificity of rules could at least make restyling easier, which could be a separate way to go if we do not enable ad placements without styles.

For now, you could also use a rule with higher specificity, like an id attribute, on the top level placement element. That would at least save you some typing.