(TTML) Allow different times on individual spans

[vevv]

Some subtitles contain elements, which have differently timed lines within one <p>. Those should probably be treated as two separate lines. Current parser just skips them, which is suboptimal. Might need to pay attention to styles, to avoid issues in conjunction with #29.

<p region="R27" style="S2" xml:id="C28">
  <span begin="00:01:59.640" end="00:02:02.560" style="S5">Why should I pay for it? No way!</span>
  <br/>
  <span begin="00:02:01.120" end="00:02:02.560" style="S3">DOORBELL RINGS</span>
</p>