Rework programmatic customization of VimFx

Rework programmatic customization of VimFx

- The public API has been removed. While the idea of allowing other extensions
  to extend VimFx is neat, I don't think it'll ever be used. KISS. As a
  replacement, the config files have become more top-notch.

- The former public API is now the config file API.

- Config files are no longer regular add-ons. We used to provide a boilerplate
  for such an add-on, which consisted of a bunch of files users most likely
  wouldn't touch as well as `config.js` and `frame.js`. Now, the concept of
  `config.js` and `frame.js` has been kept, but they are loaded directly, if the
  path to their parent directory is given in the `config_file_directory` pref.
  This is much easier and simpler, and does not require the user to install a
  version of Firefox with the ability to install unsign add-ons (or having to go
  through the hassle of actually signing the "config file add-on").

- `zr` reloads the config file(s). This is possible because of the above point
  and is much nicer than having to restart Firefox every time you change your
  config (or setting up something like the Extension Auto-Installer).

- `vimfx.send(...)` and `vimfx.listen(...)` have been added, to make it easier
  to create custom commands that interact with web page content, which is quite
  common. These are basically `messageManager.send` and `messageManager.listen`.
  To make the API simpler, instead of passing the name of the callback message
  to listeners and having to do `messageManager.send(callback, data)` to respond
  to a message, a real callback _function_ is now passed instead, which does the
  message sending behind the scenes. This also resulted in some nice code
  cleanup overall.

- `vimfx.off(...)` has been added, to remove `vimfx.on(...)` listeners. This is
  mostly used internally. Since `zr` allows reloading the config file,
  everything in it must be undoable.

- By loading `frame.js` directly, it is now possible to pass a real API to it,
  rather than relying on `VimFx*` properties on the global frame script object.

- The new, "real" frame script API now has tests. This involved adding support
  for running tests in frame scripts.

Fixes #682.