Do not automatically register plugin

[RobbieTheWagner]

Would you be open to a PR that removes the automatic registration, in favor of people importing and registering as needed?

[alexkuc]

I have created a PR of this plugin which have addressed this issue - https://github.com/alexkuc/chartjs-plugin-doughnutlabel-rebourne.

Ps. I assume that original maintainers of this repo are busy with something else; if they will become available, I am more than happy to merge my fork into this one