Allow for YouTube Subscribe Functionality

[danpastori]

Problem

YouTube subscribe is not supported.

Rationale

Adding a YouTube subscribe button would be awesome and allow users to see how many
people are subscribed to the channel, but also subscribe easily themselves. This would
work great for embedded videos.

Solution

I'll implement the YouTube subscribe button using the YouTube API. This will be dependent
upon expanding the config to allow for API keys and usernames.

Pros of Solution:

• Shr will have a button that will allow users to subscribe on YouTube. This is helpful for embedded videos.

Cons of Solution:

• Shr will handle more features, but is this something you want?

If you'd like to move forward, let me know and I'll submit a PR!

[sampotts]

Sounds good to me! Another good idea 👍