Integrate grViz figures into bookdown

[dchiu911]

I reviewed #256, #70, #207, #133 but couldn't quite find a solution.

A plot generated in a knitr chunk (e.g. by ggplot) gets saved as a figure with a filename that is passed from the chunk label, in _bookdown_files directory. The same is not done with grViz figures. To emulate this behaviour, one can do the following to both print and save a grViz figure

library(magrittr)
g <- DiagrammeR::grViz('digraph{a->b; c->a; c->b; c->d;}')
g

g %>%
  DiagrammeRsvg::export_svg() %>%
  charToRaw() %>%
  rsvg::rsvg_png("g.png")

^{Created on 2019-04-25 by the reprex package (v0.2.1)}

But this does not programatically pass the chunk label to the png file name and put it in the same directory as the other saved figures.

[pa0]

I think You have to add in text:

![](g.png)