Allow attributes injection into HTMLAsset

[vchuravy]

I would like to use a privacy conscious alternative to Google Analytics,
and for their needs I need to inject the following header:

<script defer data-domain="yourdomain.com" src="https://plausible.io/js/plausible.js"></script>

I checked that:

makedocs(;
    format=Documenter.HTML(;
        prettyurls=get(ENV, "CI", "false") == "true",
        assets=[
            asset("https://plausible.io/js/plausible.js", class=:js, attributes=Dict(Symbol("data-domain") => "yourdomain.com", :defer => ""))
	    ],
    ),
)

Does seem to generate the right HTML.