Add Pikchr support

[data-man]

No description provided.

[data-man]

The simple sample:

```pikchr
circle "lowdown"  color green
box rad 10px "+" bold
cylinder "pikchr" color blue
box "==" color 654321
oval "super cool!" color red
```

Rendered as:

[kristapsdz]

Dr Hipp is brilliant, so I like the premise of this, but the biggest issue in getting this merged is that only -thtml will have output. It should have terminal (for -tterm, -tgemini), HTML, LaTeX, and ideally PIC as well, or else it'll be just another corner case of something supported for HTML.

[data-man]

the biggest issue in getting this merged is that only -thtml will have output. It should have terminal

Of course, because pikchr only outputs SVG (at least for now). Why would SVG be useful in terminals?

But if you don't mind adding dependencies, I can add using ImageMagick to lowdown.
Then lowdown will be able to show images by converting them to sixels.

I know some Markdown viewers that support displaying images in a terminal, but they are written in Rust, and I' m not Rust friend. :)