Allow for consumers to control whether or not styles are added to cues

[evanfarina]

It is difficult and somewhat annoying to override inline styles that are being set on the cues. This PR still provides consumers with the styles that VTT feels are most accessible, but makes their application optional. Consumers, if they pass an option, will be notified via callback function when cue styles can be applied; the callback will receive a single argument, a hash of style properties to values, which it can use to set styles on its own.