screen-diff-plugin: support multiple screen diffs per test via custom attachment view

[just-boris]

Context

closes #613

Allow multiple screenshot differences via attachments. It should have the following format

{
  "expected": "data:image/png;base64,....",
  "actual": "data:image/png;base64,....",
  "diff": "data:image/png;base64,...."
}

To support this, I extended plugin API allowing to register custom attachment views per mime type

allure.api.addAttachmentViewer(mimeType, {
  View: CustomView,
  icon: 'fa  fa-exchange', // custom icon if you want
});

Checklist

• Sign Allure CLA
• Provide unit tests – where are all frontend tests? Can't find any

[just-boris]

Jenkins does not seem to publish demo report in the build artifacts. Uploaded the demo here: https://epic-villani-f00688.netlify.app

[baev]

LGTM

[just-boris]

Status update: got this working locally with allure-mocha. Some more changes are required. Will try to do them as soon as I can.

[baev]

@just-boris 👏 👏 👏