Add cssClass option to override any styles in css

[kesha-antonov]

You can pass cssClass option to add class to each clusterer and override any of properties in css with !important.
In js:

new MarkerClusterer(map, markers, {cssClass: 'my-clusterer'})

In css:

.my-clusterer {
  font-size: 40px !important
}

[XmlmXmlmX]

Same pull #24