Allow merge for expandProps option

[vincerubinetti]

I want to add a particular global CSS class name to all of my imported SVGs, e.g. svgProps: { className: "icon" }, but if I do this and I also want to pass in classNames on a per-icon basis, the latter will overwrite the global class. Setting expandProps: "start" just makes the overwrite in the other direction, i.e. global class but no per-icon class.

It would be good if there was a merge option for expandProps, so it would keep both.

[vincerubinetti]

Looks like this has an answer here:

#770