periph-play: create an interactive web ui to run adhoc programs

[maruel]

Similar in spirit to https://play.golang.org/ and BoneScript which is on BeagleBone's default image.

The rough idea looks like:

• Create a template:

  package main
  import (
    "log"
    "periph.io/x/host/v3"
  )
  func main() {
    if _, err := host.Init(); err != nil {
      log.Fata(err)
    }
    {{.UserCode}}
  }

• Inject the user's code in the template.
• Run goimports -w, found at https://golang.org/x/tools/cmd/goimports
• Send the formatted user code back to the Web UI.
• Report errors. In this case, the formatted code should be sent back with highlighting.
• Stream results back via websocket.
• Expose a Stop button that sends SIGTERM to the process.