When developing modules for Drupal, there inevitably comes a time when you may need to include an external CSS or Javascript plugin or library for your project.
Up through Drupal 7, this was fairly easy to do by leveraging the Libraries API module to define and manage dependencies. However, the 8.x version is not complete yet - but there is an easy way to carry on until a solution exists.
Recently, we released two modules for Drupal 8 that make use of external plugins as part of their integrated functionality: CodeSnippet and iGrowl.
Both modules need plugins, but we don’t want to package them into the module as there are licensing and policies prohibiting this for good reason. There isn’t a way to use Bower or Node either, and that just puts more dependencies on the end user when installing.
One accepted practice in Drupal 8 is to create a libraries folder in your Drupal root, which is where these plugins and libraries can live. As an end user, the only thing to do is to download the plugins and place them there, and you're good to go with those modules. We’ve also included instructions in the modules on doing this. Do note that the libraries folder should be outside of the /core folder.
From there, you can point your module to look for code in the libraries folder either in your classes or from your modulename.libraries.yml file. Look at CodeSnippet to see how this works.
Until the Libraries API module catches up or a better method comes along, this is how I have been solving the problem and decoupling libraries/plugins from modules.