Support MPEG-DASH Subtitle Download

[tamplin]

Please follow the guide below

• You will be asked some questions and requested to provide some information, please read them carefully and answer honestly
• Put an x into all the boxes [x] relevant to your issue (like that [x])
• Use Preview tab to see how your issue will actually look like

---

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.05.29. If it's not read this FAQ entry and update. Issues with outdated version will be rejected.

• [√] I've verified and I assure that I'm running youtube-dl 2017.05.29

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• [√] Feature request (request for a new functionality)
• Question
• Other

---

Description of your issue, suggested solution and other information

subsegmentAlignment="true" is this saied that subtitles can be downloaded ？
If so , support for downloading subtitles is an very important function .THX

[dstftw]

Carefully read new issue template and provide all requested information.