Allow the playing back of remixes

[maboa]

Remixes are effectively a playlist of clips.

The transcript markup allows for remixes to be represented with clips defined in individual <section>s.

In each section we can define a source media file and optionally* a start and end point of the clip.

If start or end time of clip not defined, we can start at the first contained word time and/or finish after the last word start (plus duration).