Add `overlay` parameter to `ImageGrid`

[SarthakJariwala]

Add ability to overlay multiple images in ImageGrid using a new overlay parameter.

g = isns.ImageGrid(IMG_LIST, overlay=True)
g = isns.ImageGrid(3D_IMG, overlay=True)
```
would result in a `ImageGrid` object with a single axes with overlayed images.

This would then also allow users to supply different colormaps for the overlay - 

```python
g = isns.ImageGrid(IMG_LIST, overlay=True, cmap=["Reds", "Blues", "Greens"])