feat: options to display continuous sliders

[Alan-Kuan]

Hello.

I like what this plugin does, but what annoyed me was that the appearance of the sliders looked uncontinuous because the background color of the points were empty.
Thus, I added some options to make it possible to display continuous sliders.
In order not to introduce breaking changes to everyone's configs, the default behavior is same as before.

The options I added:

• empty_point_bg
  • default: true
  • whether to give points background colors
• point_color_on_dark
  • default: "#ffffff"
  • point color on a dark background
  • only works if empty_point_bg is false
• point_color_on_light
  • default: "#000000"
  • point color on a light background
  • only works if empty_point_bg is false

The followings are some screenshots.

Besides, I updated the document, so you can also take a look.

Please tell me if there's anything I missed.

[uga-rosa]

As for what you want to do, I understand and I think the policy is good.

[Alan-Kuan]

@uga-rosa I finished. Please take a look.

[uga-rosa]

LGTM
Thanks for your work and contribution!