Add context menu link to 'Open with LÖVELINESS'.

* Also share more code between mimetype handler, drag+drop loader, and
  context menu loader.
* Fix some issues with the code injection
* Inject much less code by default, inject more after we know we are
  interested in modifying the page.
* Add key to manifest.json so it will have a consistent extension id,
  even when testing. This cannot be included in the final package,
  however.
Compress .zip file more, and remove it before creating
context_menu.html uses a URL query parameter.

This allows you to reload the page, bookmark it, etc.
Handle quit message.

When the user quits, we destroy the embed, and display "back" and
"reload" buttons.
Bump version to 0.2.0