Add ability to pass arguments to dvisvgm through engine.opts (fixes #2038)

[andrewheiss]

When using fig.ext="svg" with a TikZ chunk, it is now possible to pass additional arguments and flags to the dvisvgm command, like --font-format=X for embedding fonts, or --no-fonts for converting all text to outlines. A chunk like this will now convert to an SVG with embedded fonts, for instance:

```{tikz example, fig.ext="svg", engine.opts=list(dvisvgm.opts = "--font-format=woff")}
\usetikzlibrary{positioning}
\begin{tikzpicture}
\node (X) at (0,0) {$X_{it}$};
\end{tikzpicture}
```

[CLAassistant]

All committers have signed the CLA.

[yihui]

Looks good! I'll merge tomorrow or next Monday (this new feature needs to be mentioned in NEWS.md; it'll be great if you could add a bullet there). Thanks!