Add an 'engine_args' option similar to tinytex's.

[EmmanuelCharpentier]

The issue #172 reported the difficulties to use tikzDevice in a \LaTeX file using packages needing the -shell-escape option to {pdf|xe|lua}latex in order to access the filesystem and system commands.

This patch request implements a solution to issue #172. it adds an argument engine_args similar to the one present in tinytex, with a corresponding global option tikzEngineArgs.

This patch request passes R CMD check --as-cran. I have installed it on R 3.4.4 and 3.5.1.

Of note : this is not sufficient to use it transparently for use with knitr ; in order to use t transparently, you also tave to set tinytex.engine_args to the same -shell-escape value. I think that this is a bug in the way my system (emacs) invokes the various compilers, an consider it a different problem.

Home this helps...

[rstub]

Thanks for the contribution. Can you please remove the white-space / formatting changes, e.g. in R/tikz.R? That way the PR would be easier to evaluate.