Render attribution text using innerHTML property.

[adeschamps]

This makes it possible to use attributions that have links in them.

I considered whether attributes should be Html msg instead of String, but it seems like tile sources are often accompanied by a snippet of HTML for you to copy-paste. On the other hand, this approach doesn't really allow you to style the attribution, so maybe Html msg is the way to go.

[klaftertief]

I did not want to have it Html msg because I wanted to limit the number of allowed elements. To support links I head the idea to use a proper type for the attribution that allowed either just a String or a String with a URL or a combination of both. But adding plain Html and an Html as String makes also sense. Will think about the preferred way a bit. Thanks :-)