fix #122

[pachadotdev]

I added some checks and now I can create both html and pdf outputs.

However, the console says:

...
--include-in-header /home/pacha/R/x86_64-pc-linux-gnu-library/4.3/rjtools/rjdistill.html --filter /usr/bin/pandoc-citeproc 

Error: walk method does not exist

Output created
...