Restore YouTube options for custom iframe title and title download option

Closes #205. Genuinely fixes #230 instead of just mitigating it. Supersedes #224.

This PR adds several options related to YouTube embed titles:

The default iframe title attribute is now configurable as an option. The default value remains the same.
You can now opt to automatically download the actual title of the video from YouTube. This can be an accessibility win, since the iframe will better reflect the real content of the video. Note that activating this feature will cause network calls to YouTube for each embedded video — on large sites this could result in slow builds, or rate-limiting by YouTube, or both. If there's any failure in fetching the title, it will fall back to the default value.

How to use it

This simplified example shows the new title options in action:

eleventyConfig.addPlugin(embedYouTube, {
  title: "Custom iframe title wooooo",    // Default/fallback iframe title value
  titleOptions: {
    download: true,                       // Download the titles from YouTube
    cacheDuration: '1h'                   // Cache titles for one hour
  }
});

Notes on implementation

Javascript’s native .replace prototype is synchronous, which makes awaiting the title impossible, so we now use the string-replace-async package instead.
The nature of this update makes it kind of an all-or-nothing change; the YouTube plugin is asynchronous for everyone as of this release, whether they turn on titleOptions.download or not.
One nice effect is that making the pattern-replacements async is that they can also be made concurrent, so that could be a performance win.
I've added some new tests just to really convince myself that the new behavior is truly equivalent to .replace(). It seems to be at parity.

gfscott / eleventy-plugin-embed-everything

Restore YouTube options for custom iframe title and title download option #233

How to use it

Notes on implementation

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