Change `chart.js` to a peer dependency

[kiprasmel]

This allows the user to install the preferred chart.js version

For example, I needed some bugfixes from upstream.

The user would obviously need to install chart.js himself, so this probably is a breaking change?

---

Edit:

It doesn't seem like this is going to be fixed.

For anyone looking for a work-around - I've started using yarn over npm, because it can do something like this, called dependency resolution:

package.json

{
    "dependencies": {
		"angular2-chartjs": "^0.5.1"
	},
    "resolutions": {
        "angular2-chartjs/chart.js": "^2.8.0"
    }
}

as you can see - you can modify deep dependencies' versions and such.

Once again - check the documentation.

Also, I'm aware that npm has a package npm-force-resolutions, but this feature is not natively supported by npm itself. And yarn turned out pretty cool for me, so I didn't mind the switch.

P.S. - I've even tried generating patches for the deep dependencies' package.json, but it turns out that npm adds a lot of junk that makes the patches unusable, since the changed content differs everytime you re-install dependencies, like after cloning the project. See https://github.com/ds300/patch-package/issues/150 for reference.

( I've even tried making a custom postinstall script for npm to run, but turns out that npm only applies the modifications to package-lock.json file only AFTER the whole installation, including the postinstall script, has finished, effectively meaning that's it's either impossible or really hard/complicated/not worth to do. )