feat: added the ability to set the size of an icon when using a dot s…

[jamespcole]

This is a small change allowing the icon size to be scaled relative to the node when using the dot shape.

The existing "icon.size" property is ignored when using a "dot" shape so this new option is intended to enable specifying the icon size relative to the shape size.

To use this a "scale" setting can be added to the icon settings:

{
    shape: 'dot',
    size: 30,
    icon: {
        face: 'FontAwesome',
        color: '#2B7CE9',
        scale: 0.9,
    }
}

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[exoego]

Code is LGTM.

It would increase a chance to get merged if new option is added to

• docs: https://github.com/almende/vis/blob/5563fc616885578ad51b2dfa6ccf70ccebf06126/docs/network/configure.html#L133
• examples: https://github.com/almende/vis/blob/master/examples/network/nodeStyles/icons.html