Adding support for css class styling.

[KevinJCandlert]

I wanted a way to get custom styling from CSS instead of having repeatedly inline CSS for each cluster marker.
If the cssClass options are supplied it will ignore the default styling and the options from the 'styles' parameter.

This is more or less the same feature as #24 but with less white-space changes and different implementation.

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[KevinJCandlert]

I signed it!

[Remo]

@KevinJCandlert google doesn't support this anymore, but we've created a fork where you can find this option https://gmaps-marker-clusterer.github.io/gmaps-marker-clusterer/#reference-styles

[KevinJCandlert]

@Remo Yeah, I've noticed that. But I couldn't find which branch you've merged.
I'm not entirely satisfied with my own merge request - As it ignores all the other styling options.

But just adding a CSS class to the cluster-marker along with all the inline styling requires you to add !important to all your CSS-attributes.

It's hard to get it right and also maintain some kind of good UX for the API 👍

[Remo]

I think there were two branches, this should be the one that was merged (not by me) https://github.com/gmaps-marker-clusterer/gmaps-marker-clusterer/pull/6/files