Add line color hue shift speed

[Crul]

Added an option to dynamically change the hue of the line color.

Example of what can be done: https://i.imgur.com/18sMupc.png
Config for that example: #r580ro-0.9rot0.41rota0.59e0.995ex0.996t0.7se6700sk0.04i2000w1200h1200c10ca15can18l255li142lin78line0.49b6f788rotat822wa8508wav3015wavi1.3j0.3linec2hu0.42

Comments:

• new hueshiftspeed option added at the bottom (following the guidelines)
• option ranges (-10 to 10, step 0.1) chosen by eyeballing
• changeHue function adapted from here https://stackoverflow.com/a/17433060
• changeHue and auxiliary functions added bellow other auxiliary functions, between rotate and restartRendering
• letter for the UI slider (ܘ) chosen at random from the (unused ones of the) Aramaic alfabet
• performance wise, when hueshiftspeed != 0 it would be better to store the initial color value as HSL to avoid the RGB->HSL conversion each step. Or even avoiding RGB and specifying the value as hsla(...). I didn't see it worth the effort.

[doersino]

This is a great addition! It creates a very psychedelic (or, pro-LGBT) vibe, which is interesting to play around with.

I've pointed out a few very minor issues (this is my first time trying out GitHub's built-in code review tooling), but it's honestly a great pull request! Thank you very much.

[doersino]

Awesome, thank you so much for addressing my nitpicky requests for changes!

I'll go through the presets and check which ones can be improved with a subtle hue shift.

[Crul]

Yeah, I preferred not to touch the presets, it is better that you choose the values you like, my taste for colors is not the most refined in the world.

Thanks!

[doersino]

Made a small change, hope you don't mind. 😉

[Crul]

Haha, it fits perfectly!