dynamically show/hide markers

[Remo]

Didn't sign a CLA yet and code isn't super nice, but still posting this in case others run into the same problem. This approach seems to work so far https://github.com/googlemaps/js-marker-clusterer/issues/31

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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[Remo]

I signed it!

[googlebot]

CLAs look good, thanks!

[Remo]

sorry, still has a problem, the count shown on the cluster is wrong!