Added customized className for rows

[gouruiyu]

Motivation:
When working with dat.gui, I encountered this use case where there are two set of markers with different colors to show(screenshot attached). The border left bar seems to be a good place for color indications. However, the color scheme for the gui seems to be quite fixed. Thus I modified the code to allow customized className when adding a controller row, I can then overwrite the style in customized CSS.

Use case:
Javascript:

Folder1.add({ marker1: false, className: "markerGreen" }, 'marker1')
    .name('Marker1')
    // everything else remain unchanged...

CSS:

  .dg .cr.markerGreen{
    border-left: 3px solid #00ff00 !important;
  }

I think it's worthwhile to allow some freedom w.r.t the color/style, but not sure if it's the best way to go. Any suggestions would be appreciated!

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[gouruiyu]

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