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
