Add support for custom A-Frame Watcher save endpoint location (#709)

[bcopy]

• Allow users to register a custom A-Frame Watcher save endpoint URL, e.g. :

  <script>
    window.AFRAME_WATCHER_URL = "http://my-custom-host.org:1234/save";
  </script>

• Patch the scenegraph toolbar SAVE button to use the custom endpoint URL.

[vincentfretin]

There is a feature that was asked in #709 to also save to another endpoint, not necessary aframe-watcher endpoint.

Can you please remove from the PR the changes in dist and package.json.

We now have the AFRAME_INSPECTOR_CONFIG to set configuration, so you should use it instead of introducing a new variable.
I suggest:

window.AFRAME_INSPECTOR_CONFIG = { saveEndpoint: "http://my-custom-host.org:1234/save" };

You can put the default value in lib/config.js as well.
You then access the config with AFRAME.INSPECTOR.config.saveEndpoint

I would also make the tooltip text generic, instead of "Write changes with aframe-watcher." just say "Save changes".

I'd probably should put pack the save button on aframe-editor, I clearly see some of you are using the feature.