use standalone class internal conversion to png

[yonicd]

There is also a convert option now (to png) in section 5.6 of the documentation http://mirrors.ibiblio.org/CTAN/macros/latex/contrib/standalone/standalone.pdf.

This would cut out a lot of the fat from the package.

It uses imagemagick too under the hood, but would be better integration and less R side package dependency.

[yonicd]

keeping magick as imagemagick engine