Adds Pikchr support to VS Code's built-in Markdown preview
Usage
Create diagrams in markdown using pikchr fenced code blocks:
```pikchr
color = white
linerad = 10px
linewid *= 0.5
circle radius 10%
arrow 2*arrowht
oval "(" bold fit
arrow 2*arrowht
arrow
CN: oval "column-name" fit
arrow 125%
oval ")" bold fit
arrow 2*arrowht
circle same
CM: oval "," bold fit at 1.25*CN.ht below CN
arrow from CN.e right 2*arrowht then down even with CM then to CM.e
arrow from CM.w left even with 3*arrowht west of CN.w \
then up even with CN then to CN.w
```
```pikchr
color = white
arrow right 200% "Markdown" "Source"
box rad 10px "Markdown" "Formatter" "(markdown.c)" fit
arrow right 200% "HTML+SVG" "Output"
arrow <-> down 70% from last box.s
box same "Pikchr" "Formatter" "(pikchr.c)" fit
```
