Deduplicate active plugin list when using 'add' option

[gfscott]

I spotted someone in the wild with a config object like this:

eleventyConfig.addPlugin(embedEverything, {
  add: ['twitter', 'soundcloud', 'youtube']
});

The original intent behind the add option was to make it easier to activate non-default plugins (of which there's currently only one, soundcloud). I tried it out and discovered that this usage results in twitter and youtube showing up twice in the active plugin list. The effect probably isn't noticeable to the end user but this PR ensures that the list of active plugins is always deduplicated.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1fd1ba8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eleventy-plugin-embed-everything	Patch

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