Adding support to WebVTT (.vtt) to babelsubs.

Based on what I found inside the tree, I implemented: 

- WEBVTTGenerator
- WEBVTTParser
- Test cases for WEBVTT.

Thanks to the earlier contributors who made this task easier, with proper comments and great coding. Much of it is based on SRT, changing what is necessary to make it WebVTT compliant.