Add support for dynamic existing-color-handling to all effects.

[ChrisBuilds]

The --existing-color-handling argument supports a dynamic value which enables a per-effect implementation of color sequence handling. Each effect must be updated to support this setting.

• [x] Beams
• [ ] Binarypath
• [x] Blackhole
• [ ] Bouncyballs
• [ ] Bubbles
• [ ] Burn
• [ ] Colorshift
• [ ] Crumble
• [ ] Decrypt
• [ ] Errorcorrect
• [ ] Expand
• [ ] Fireworks
• [ ] Highlight
• [ ] Matrix
• [ ] Middleout
• [ ] Orbittingvolley
• [x] Overflow
• [ ] Pour
• [ ] Print
• [ ] Rain
• [ ] Randomsequence
• [ ] Rings
• [ ] Scattered
• [ ] Slice
• [ ] Slide
• [ ] Spotlights
• [ ] Spray
• [ ] Swarm
• [ ] Synthgrid
• [ ] Unstable
• [ ] VHSTape
• [ ] Waves
• [ ] Wipe