Add webpquality support for renditions

[ruisaraiva19]

Resolves #125

By default, all bmp and webp images are converted to the png format when no image output format is given.
The default conversion mapping can be changed (docs here).

{
  image(id: 2) {
    id
    url
    rendition(webpquality: 75) {
      url
    }
  }
}

{
  "data": {
    "image": {
      "id": 2,
      "url": "http://localhost:8000/media/original_images/file_example_WEBP_50kB.webp",
      "rendition": {
        "url": "/media/images/file_example_WEBP_50kB.webpquality-75.png"
      }
    }
  }
}

[zerolab]

LGTM.
Need to check how Wagtail tackles this, but I wonder what happens when you pass webpquality to a jpeg and viceversa 🤔

[zerolab]

48d2e1b makes me think we are missing some tests - would be good to get an idea of the type of query/queries that made you discover the inconsistency.

Nonetheless, let's get this in and follow up with ^