#3 Support for external pgf/TikZ graphics not in any chunk

[Cameron Bracken]

When a pgf/TikZ graphic is included manually (not in a code chunk) add the commands to externalize into the graphics into the shell script that is created. This may need to be a separate function that gives the tex file a once over and checks if the pdf file exists corresponding to the name in \beginpgfgraphicnamed{name} or the corresponding TikZ externalization command. There are already scripts available on example to do stuff like this so maybe integration of one of those may be possible.