Add Group Verfications

[nitrag]

Thanks for writing this plugin!

I've made enhancements so that we can also verify that the user is in a specific group: "3DPrinters" for an extra level of access control.

[gillg]

Many thanks for your enhancement ! It's a very good thing, and your code is OK for me.
I let your PR in stand by, because I work on a paralelle branch (see #3) to make this plugin more "standard" and add working config template.
I will see to merge now and rebase other branch or add your code after.

[gillg]

All is OK for me ;)