Add getTikzDefaults function

[kylebutts]

Add a getTikzDefaults function. My use case is get default tikzLatexPackages, add a package, and set as option:

library(tizkDevice)
setTikzDefaults()
default_packages <- getOption("tikzLatexPackages")
options("tikzLatexPackages" = c(default_packages, "\\usepackage{bm}\n"))

becomes

library(tizkDevice)
options("tikzLatexPackages" = c(getTikzDefaults()$tikzLatexPackages, "\\usepackage{bm}\n"))

[kylebutts]

Hi @rstub, would you mind giving this a review? Should be quick and easy :-)