Consider adjusting license from "GPLv3" to "GPLv2 or later"

[GaryJones]

For the future, it would be worth getting approval from all the contributors so this could be licensed under "GPLv2 or later", which is standard for plugins, and used by WP core.

As noted at https://www.gnu.org/licenses/license-list.en.html:

Please note that GPLv3 is not compatible with GPLv2 by itself. However, most software released under GPLv2 allows you to use the terms of later versions of the GPL as well. When this is the case, you can use the code under GPLv3 to make the desired combination.

My understanding of what this means is that if someone is running a plugin that only has GPLv2, they can't then use this GPLv3 in the same site. By switching licenses, we provide as much leeway for sites to use this with other plugins.