Make scaling factor of selected edges configurable via widthSelectedScale

It would be nice if the highlight settings allowed disabling/configuring the width scaling factor in Edge.prototype._getLineWidth, which is used when a node/edge is selected.

I'm currently using highlight colors to indicate the edges connected to the selected node. The hardcoded width scaling factor of 2 makes the resulting graph look ugly :-(

Couldn't figure out a better/shorter name for the option.

PS. While this is a small change, I haven't written much JS so it's probably worth an extra check that all is OK.