Links to single plugin PHP file should work with `plugin install`

[schlessera]

When using wp plugin install <URL>, WP-CLI currently assumes that the URL is pointing to a ZIP file of a plugin.

This command should also work with a URL to a single PHP plugin file, so that it can for example directly install plugins from GitHub Gists:

Example of such a command that should just work:

wp plugin install --activate https://gist.githubusercontent.com/westonruter/dec7d190060732e29a09751ab99cc549/raw/d55866c2fc82ab16f8909ce73fc89986ab28d727/pwa-manifest-short-name.php

/cc @westonruter (whose numerous Gist plugins make me want to implement the above)

[westonruter]

What about if a Gist URL is provided, like: https://gist.github.com/westonruter/dec7d190060732e29a09751ab99cc549

I'd expect that to just work. It should fetch the ZIP for the Gist, determine the slug of the plugin from the name of the plugin file, and then create a directory for that plugin using the slug. The content of the Gist can then go in there. This is important because Gists are not limited to single files. For example: https://gist.github.com/westonruter/26667d26ea34ef8e9f5e1053a747c73f

The above is what the GitHub Updater plugin is doing: afragen/git-updater#881 (comment)