[FEATURE] Ability to add subscription value via config (#744)
Subscription Value is the mechanism used to achieve one-liner subscriptions. It works by having the subscription "Subscription Name": "value", and assigning "value" to an override variable via setting the subscription value to that override variable's name.
This feature allows you to set subscription value within the config. In the near future, ytdl-sub will strive to make subscription files simpler by hiding most of the "under-the-hood" logic within the config file.