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
Middleware registration now only accept a factory method which takes a player reference and returns some object that represents the middleware with the various methods on it.
Also, add a use to register a middleware for all types.
Add middleware support. Middleware can function as go-between between the player and the tech. For example, it can modify the duration that the tech returns to the player. In addition, middleware allow for supporting custom video sources and types.
Currently, middleware can only intercept timeline methods like duration, currentTime, and setCurrentTime.
For example,
```js
videojs.use('video/foo', {
setSource(src, next) {
next(null, {
src: 'http://example.com/video.mp4',
type: 'video/mp4'
});
}
});
```
Will allow you to set a source with type `video/foo` which will play back `video.mp4`.
This makes setting the source asynchronous, which aligns it with the spec a bit more. Methods like play can still be called synchronously on the player after setting the source and the player will play once the source has loaded.
`sourceOrder` option was removed as well and it will now always use source ordering.
BREAKING CHANGE: setting the source is now asynchronous. `sourceOrder` option removed and made the default.
