Make dependencies more modular

[GregorDeCillia]

Make the following features and their dependencies optional

• write_xlsx: Dependencies: xlsx, Sys-dependencies: Java
• write_pdf, knit_print: Dependencies: none. Sys-Dependencies: latex, and certain CTAN packages
• write_png, knit_print_html: Dependencies: All dependencies for "pdf export" as well as magick and pdftools

Also make sure each of the three modules has a descriptive error function if the dependencies are not available. For example

write_pdf(st)
#> Error in write_pdf: 
#>   Could not find pdflatex on your system. Please refer to the "installation" 
#>   section of vignettes("styledtables-latex") for details
write_png(st)
#> Error in write_png: 
#>   Could not find the R Package magick on your system. Please install with 
#>   install.packages("magick")
install.packages("magick")
write_png(st)
#> Error in write_png: 
#>   Could not find the R Package pdftools on your system. Please install with 
#>   install.packages("pdftools")
write_xlsx(st)
#> Error in write_xlsx:
#>   Could not find the R Package xlsx on your system. Please install with
#>   install.packages("xlsx")
":("
#> [1] ":("

The "styledtables-latex" vignette (#8) therefore needs a installation section at the beginning that documents how latex and the required CTAN packages can be installed. The pimary recommendation for this should be tinytex, since it is platform independent and CTAN packages can be installed on the fly. However, it should also be pointed out that a "normal" tex installation works as well.

[GregorDeCillia]

We should consider to use tools::texi2pdf("table.tex") instead of system("pdflatex table.tex") because of the error handling it provides. In theory, we could just replace the "error latex not found" message thrown by tools with the vignettes("styledtable-latex") message mentioned above and forward all other error messages to the client as-is.

Last time I checked, texi2pdf didn't work on appveyor but I am pretty sure the latex compiler was not properly installed on that point.