v0.9.0 -- Widgets, DOM, Wasm, Front-end communication, `nbexec`

This is a large update, which includes refactoring of internal package to an internal subdirectory:

• Added widgets support (experimental):
  • a websocket opened from the front-end that communicates
    to the kernel, and through it to the users cells.
  • API to use it in gonb/gonbui/widgets.
  • API to communicate with front-end in gonb/gonbui/comms.
    • Listen[T](address) function added to create a channel listening
      to front-end updates.
    • Added --comms_log flag to add verbose logging to the Javascript console.
  • API to manipulate the DOM in gonb/gonbui/dom.
    • Persist() added to persist transient changes to the DOM -- meaning
      dynamically generated HTML and widgets will show up when exporting
      to HTML or when running nbconvert.
• Added gonb/cmd/nbexec to execute notebooks for integration testing --
  nbconvert --execute was not working.
• Added "%wasm" support (experimental):
  • Allows compiling cell to WASM and running that in the notebook. One
    can write widgets like this. Experimental: there are some use cases are not 100% clear. See
    "%help" for details on how this works.
  • Added also github.com/janpfeifer/gonb/gonbui/wasm library with some basic helpers to write
    WASM widgets.
  • Added gonb/examples/wasm_demo.ipynb with a couple of examples of Wasm.
• Improved logging of errors; pre-checking for duplicate package, with improved error message.
• Handle incoming messages in a separated goroutine, so asynchronous/concurrent messages can be
  handled. Specially important when executing cells that take a long time.
  • Serialize "execute_request" and other "busy" type of requests, so they are executed in the order
    they are received.
• Added support for %env VAR=VALUE syntax as well (like ipython uses).