FEATURE: generate which doesn't overwrite existing

还没有人认领这个 Issue。

I have a minor hassle when adding new tests and running
```
--mpl-generate-path=tests/baseline
```
I overwrite all existing baseline images in my repo. I then have to sort through which are added, because usually the regenerated old tests are still the same visually, but are not identical hash-wise.

I would propose adding a
```
--mpl-generate-missing=tests/baseline
```
Which would only generate new figures if the baseline is mising.

首先跟踪 --mpl-generate-path=tests/baseline 的处理方式，以及基线图像的生成位置。将该流程与仅生成缺失图像的拟议行为进行比较；当现有基线图像保持不变，同时缺失的图像在指定路径中生成时，即表示完成。

由索引模型根据 Issue 内容生成。