[Feature Request]: df.plot(type='heatmap')
A heatmap is a way of visualizing a matrix: 
![image](https://user-images.githubusercontent.com/2761597/34457687-079c7fce-ed87-11e7-8c29-3e778347fe70.png)

It's particularly common in biology / bioinformatics. It makes for a good way to begin to form hypotheses about the latent structure in some data. 

There exists a library which can produce heatmaps in python, specifically [seaborn](https://seaborn.pydata.org/generated/seaborn.heatmap.html). However, the fact this feature already exists in another library doesn't necessarily mean it shouldn't exist here -- after all, both libraries provide bar plots, scatter plots, etc... And secondly, the seaborn implementation is very slow relative to R's heatmap plotting, so there's room for a faster implementation in the python world. 

I'd love to see a new, faster heatmap implementation added to pandas. Would make my life much, much easier. 