add support for coloring using CSS variables

[McShelby]

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This PR adds support for coloring the scrollbar via CSS variables.

A demo is included within this PR in examples/colors.html which was derived from examples/basic.html.

I was in need to support a dark mode variant of my project but coloring of the scrollbars did not fit. Instead of redefining all the selectors to set the CSS color attributes within my project, I wanted a more robust solution.

This fixes #15, #102

Now colors are easily modifiable by CSS variables if the browser supports them. If the browser does not support CSS variables it defaults back to the old mechanism, making it compatible with at least IE11.

To set the colors simply add the following snippet to your project's CSS

:root {
  --ps-thumb-color: #faa;
  --ps-thumb-hover-color: #f99;
  --ps-rail-hover-color: #fee;
}