setSource is useful if you care to be fiddling with the source or doing some work depending on what source is set. However, sometimes, you don't need a setSource and want the middleware to always be selected.
Now, if setSource is missing, it will implicitly be included in the middleware chain.
This will allow middleware to interact with calls to play() from the tech. This will require a method of indicating to middleware previously run that a middleware down the chain has terminated or stopped execution.
* Adds middleware mediator method that runs middleware from the player to the tech and a second time back up to the player. This category was created because play is both a setter(changes the playback state) and a getter(gets a native play promise if available). This also has the ability to tell whether a middleware has terminated before reaching the tech.
* Adds a middleware.TERMINATOR sentinel value that is available on the videojs object
* Adds play to the allowedMediators
* Adds paused to the allowedGetters
* Adds a sandbox example of a play mediator middleware
