feat(inline): append new classes to existing ones

[sagikazarmark]

Description

When inlining resources with attributes (eg. SVG), classes will now be appended to an existing list of classes (instead of being ignored).

Related Issues

Fixes #721

Check List

• Have you read the
  CODE OF CONDUCT
• Have you read the document
  CONTRIBUTING
  • One pull request per feature. If you want to do more than one thing,
    send multiple pull request.
  • Write tests.
  • Run deno fmt to fix the code format before commit.
  • Document any change in the CHANGELOG.md.

[sagikazarmark]

deno fmt formats code that I haven't touched, so I skipped running it. Code changed by this PR seems to follow the formatting rules.

[oscarotero]

Probably it's some change in the formatter introduced in the latest version of Deno.
Anyway, your code looks great, so I'm going to merge it and will re-run the fmt to the whole code repo.