Adds WebVTT and expands splitting functionality

* Adds a webvtt_time_str method in Line to allow WebVTT style timestamps;
* Adds a to_webvtt method (analogous to to_s and using that as a base) to File that adds the WebVTT header and uses the WebVTT style timestamps;
* Adds an option to split without renumbering the lines;
* Adds an option to splt at regular intervals instead of based on a list of split points.

What we have at the end of this is the ability to output not only SRT but also WebVTT, as well as an easy to use segmenting functionality for both SRT and WebVTT.