Support importing remote plugins

[ianjennings]

The current repl uses http://bundle.run to import plugins. This is really cool, but it doesn't include configs and we're not huge fans of the current design for this.

So we need:

• A button to trigger a modal that displays
  • A list of "official plugins", display the list found here (keep headings): https://babeljs.io/docs/en/plugins/
  • The ability to enter an npm package name into a textbox
• A submit button

The button can probably be added to the top "file menu" in the header. Something like "Add Plugin."

The current babel repl has: