## Description
It is useful to have methods for appending and removing `<source>`
elements to the `<video>` element, as they are sometimes required to
enable certain playback features, for example, using [Airplay with
MSE](https://webkit.org/blog/15036/how-to-use-media-source-extensions-with-airplay).
## Specific Changes proposed
Add new methods-- `addSourceElement()` and `removeSourceElement()` to
the player and tech. The former will take a source object and create and
append a new `<source>` element to the `<video>` element, and the latter
will take a source url and remove any `<source>` element with a matching
`src`.
## Requirements Checklist
- [ ] Feature implemented / Bug fixed
- [ ] If necessary, more likely in a feature request than a bug fix
- [ ] Change has been verified in an actual browser (Chrome, Firefox,
IE)
- [ ] Unit Tests updated or fixed
- [ ] Docs/guides updated
- [ ] Example created ([starter template on
JSBin](https://codepen.io/gkatsev/pen/GwZegv?editors=1000#0))
- [ ] Has no DOM changes which impact accessiblilty or trigger warnings
(e.g. Chrome issues tab)
- [ ] Has no changes to JSDoc which cause `npm run docs:api` to error
- [ ] Reviewed by Two Core Contributors
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