Add compatibility with ChartJS v3

[MartijnVerkleij]

Hello there! I need to be honest with you, I have not tested this code at all, but this is essentially what came out of my attempts to get the plugin working again. So, it may only bring back rudimentary compatibility.

I have used code from someone else as you can see. I need to get in touch to confirm that this is okay. I have just received confirmation that this is not an issue.

Fixes #24
Fixes #25

[MartijnVerkleij]

Apologies for the messy commits whilst trying to fix the Travis pipeline 😊

[dregad]

Dear @nagix it would be awesome if you could merge this and publish a new release of this excellent chart.js library ! Thanks in advance.

[pkunszt]

yes please @nagix, this would make the plugin usable again with the latest version of chartJS. Thanks!

[DmytroLitvinov]

Hi @nagix ,
Thanks a lot for that plugin. It would be nice if we can keep alive that plugin :)

[mano3m]

This looks good. Could someone create a minified version of the updated plugin? Thanks!

[blikblum]

I just tested and those changes are not enough to get working with chartjs v3.

I created a working branch at https://github.com/blikblum/chartjs-plugin-colorschemes/tree/v3

Time allowing i will do a proper PR

[nagix]

@MartijnVerkleij Thank you for your contribution! This looks good to me.