return already created player when given an element

[gkatsev]

As part of the tech 2.0 work, we decided to remote the player out of the tech. One of the changes in it nulled out tag.player. This is an issue because videojs(el) uses tag.player to see whether we have already created, so, if you try to get the player again with the same el, it'll actually create a brand new player for you.
An example:

let el = document.querySelector('#my-video');
console.log(el.id); // "my-video"
let player = videojs(el);
console.log(el.id); // "my-video_html5_api"
let anotherPlayer = videojs(el);
console.log(el.id); // "my-video_html5_api_html5_api"
console.log(player == anotherPlayer); // false

The expected result should be something like this:

let el = document.querySelector('#my-video');
console.log(el.id); // "my-video"
let player = videojs(el);
console.log(el.id); // "my-video_html5_api"
let anotherPlayer = videojs(el);
console.log(el.id); // "my-video_html5_api"
console.log(player == anotherPlayer); // true

There are two ways I can think off right now to fix this:

1. revert the changes that nulled out tag.player
2. when given an element and tag.player in null, we would want to parse out the id to remove the stuff like html5_api and then return videojs.players[parsedId(tag.id)] if it exists, otherwise actually go ahead and create a player

I feel like solution number 2 is more correct but it is also a lot more complicated and more prone to issues. Included below right now are two failing tests for the above usecase.
@videojs/core-committers @dconnolly thoughts?

[heff]

Since the video tag is the only tech that doubles as an embed code, you could look at it from the embed code side and add tag.player in the player init, just for this purpose.

[gkatsev]

In a conversation with @heff, we came to a simpler solution than 2 without resorting to 1. Instead, we're going to attach the playerId to the tag and use that to get back player references.

[heff]

lgtm!