Expose latex command tool chain

Dvi isn't compatible with quite a few Latex packages. I have been trying to include chemical formulas with the chemfig package. Exposing the tool chain to plugins would be very useful. For the record, I'm currently using:

```python
latexCmds = [
    ["pdflatex", "-interaction=nonstopmode", "tmp.tex"],
    ["pdflatex", "-interaction=nonstopmode", "tmp.tex"],
    ["pdfcrop", "tmp.pdf"],
    ["convert", "-density", "300", "tmp-crop.pdf", "tmp.png"]
]
```