New features:
* `--mirror-left` causes most operations to be mirrored for the left hand, including inverse offset/rotation and spiral/spike directions
* `--stack-duration=<beats>` / `--stack-duration=<seconds>s` as alternative to `--stack-count=<num>` that automatically calculates the number of stacks to fill the duration
* `--wall-rotate=<angle>`, equivalent to `--rotate=<angle> --relative -f walls`, but can be used in parallel to absolute rotation during stacking
* `--autostack` (defaults to OFFSET) / `--autostack=<mode>` with `OFFSET`, `SPIRAL`, `OUTSET` and `SCALE` modes, to automatically determine offset/rotation from two placed objects, and continue the pattern
* OFFSET uses naive xy offset, generating strictly linear stacks
* SPIRAL automatically determines a pivot point and rotation amount and generates strictly spiral-shaped stacks
* OUTSET is based on rotation and distance from center/pivot, generating cylinder or cones
* SCALE is based on relative movement, generating exponential funnels