Bug fixes, and preparation for gonb-echarts

[janpfeifer]

• Interrupt and Shutdown:
  • [interrupt_mode] set to message, as opposed to having a SIGINT. Works both in JupyterLab and VSCode.
  • Interrupt all cell executions at shutdown_request.
• New github.com/janpfeifer/gonb-echarts to add support to Apache ECharts using github.com/go-echarts/go-echarts.
• Updated documentation on VSCode limitation for Javascript.
• Fixed bug in dom.LoadScriptOrRequireJSModuleAndRun where plotly source was hardcoded by mistake.