Consider advanced facilities to include figures from other documents

[humburg]

Consider introduction of syntax to include figures generated by child documents. Currently this can be achieved by doing something along the lines of what is sketched below.

---

Child document 'child.Rmd':

'''{r prepFig}
## assemble ggplot object in a cached chunk
fig <- ggplot(...) + ...
'''

---

Parent document:

params:
    depends:
        value:
             child: child.Rmd

...
Other content
...

'''{r plotFig, dependson='child/prepFig', fig.cap="Some caption"}
plotMD(fig)
'''

---

This feels more like a workaround than an actual feature. A slightly simplified (and more obvious) way to do this might be useful.