feat: add default text color and background color classes (#34)
feat: add default text color and background color classes (#34)

This adds support for default style classes for vtt:

```vtt
00:00:00.000 -> 00:00:05.000
<c.red.bg_green>This text is red with a green background
```

As per section 5 of the spec: https://www.w3.org/TR/webvtt1/#default-classes

gkatsev/vtt.js,gkatsev/vtt.js