[Enhancement] Introduce express API #46

Thank you for maintaining ideas of keras-vis 🙏

I'm currently trying to inspect my CNN and it seems like here is the minimal amount of code it's needed to plot feature maps:

It seems to be possible to shortcut. I propose to introduce an express API for solving the most frequent use cases like:

• plot all/selected feature maps of a convolution layer
• plot all/selected filters of a convolution layer
• plot dense layer activation
• plot a class activation
• plot silency map

Sklearn and Yellowbrick have great examples of express API:

• https://github.com/scikit-learn/scikit-learn/blob/4afd4fba6/sklearn/metrics/_plot/confusion_matrix.py#L165
• https://www.scikit-yb.org/en/latest/oneliners.html

This would dramatically reduce learning curve and increase adoption of the library.