Might be useful to implement `VisualBackProp` and `DeepVis`?

Hi, I just noticed that there are some other CNN visualisation methods, e.g., "VisualBackProp" described in this paper: [VisualBackProp: efficient visualization of CNNs](https://arxiv.org/abs/1611.05418) and [DeepVis](http://yosinski.com/deepvis)

Since I am a regular user (and a big fan of the toolkit you have built, so easy to use, just pulling in the trained keras model :) ), I thought I would just put this suggestion out and hope `innvestigate` to be even more encompassing. Not sure how good this VisualBackProp/DeepVis method is (I am yet to try it myself using the repositories available online), but these videos sure look interesting: [VisualBackProp](https://www.youtube.com/watch?v=Vah7yQ4vIc8)
[DeepVis](https://www.youtube.com/watch?v=AgkfIQ4IGaM)