cli: --board flag

[alixander]

This should allow exporting a single board from a multi-board diagram.

d2 --board root.x index.d2

index.d2:

a -> b
layers: {
  x: {
    c -> d
  }
}

[Karreg]

Count me in for that one!
My use case is a technical documentation, when you want only one diagram with more or less details depending on the location in the documentation. Combined with mkdocs and mkdocs-d2-plugin it would be great!

[landmaj]

This was surprisingly easy because D2 already supported rendering a specific board - it was used in --watch mode. I opened a PR.