A better markdown builtin cell

[asg017]

Currently, Dataflow uses htl for the html and svg builtin cells, which offers much more functionality than the current builtin html and svg cells in the standard library.

However, it still uses the default markdown builtin md, which I find a little bit awkward. I'm not too sure how to improve it, but here's a few things I've always found annoying with the md builtin, which would be nice to fix:

// #1 tables

md`|These|columns|are|annoying|to|type
|-|-|-|-|-|-
${data.map(d=>{
  returning a string that gets concatenating into markdown which compiles to html, how weird
  return `|${d.i}|${d.hate}|${d.this}`
} )}`

// #2 no plugins, like checkboxes or anything

md`todo list

- [ ] this don't work
${customCheckbox("you have to build your own custom html component")}
`

Then again, markdown parsing doesn't sound too fun, it would probably be backwards incompatible, and doesnt feel too high of a priority