New feature for symmetic colormaps on images with negative values

[eugenioLR]

I have been using this library a lot and one small inconvenience i find all the time is when i want to visualize images with positive and negative values, where i use a color map like "seismic" expecting to find positive values in red and negative values in blue, but the range of values detected don't align.

To put a practical example, here i'm trying to visualize a covariance matrix.

Using the imgplot indicating only the color map i get the following image:
isns.imgplot(cov, cmap="seismic")

To fix this, i have to specify the range of values i want to show

vmin = -np.abs(cov).max()
vmax = np.abs(cov).max()
isns.imgplot(cov, cmap="seismic", vmin=vmin, vmax=vmax)

It's not that much of an issue, but it gets a bit more cumbersome when I'm showing a grid of images.

It could be a good idea to add a flag to imgplot and other functions to handle this type of images.

[eugenioLR]

I forgot to add that if this seems useful i can implement it myself.

[SarthakJariwala]

Yeah, I agree with what your suggesting - It would be useful to support it without explicitly setting vmin and vmax.

Something similar to robust=True flag. Maybe diverging=True...? Other name suggestions?

PRs are definitely welcome! Feel free to take a crack at it.