Replace the player's default icons with custom icons.

The `experimentalSvgIcons` option accepts a `boolean` or a `string`.
If the string represents a valid svg, it will replace the default icons.

**Usage**

```javascript
import playerIcons from './asset/vjs-sprite-icons.svg';

const player = videojs('my-player', {
  experimentalSvgIcons: playerIcons,
  // ...
});
```

Or:

```javascript
const player = videojs('my-player', {
  experimentalSvgIcons: `
  <svg>
  ...
  </svg>
  `,
  // ...
});
```

- extract a `initSvgIcons_` function to activate svg icons
- add test cases