feat: add option to break long lines into two

[nogarcia]

Closes #1

Sorry for taking so long! The problem turned out to be a little more (or less) complicated than I thought.

The approach I take here is to get the middle of the string (so that the lines are roughly even) and decrement that slice until it hits a space. This works well but makes some choices I'll list in case you want to change them:

• lines are only broken into two lines, rather than three (Netflix has a maximum of three)
• the end of the first line doesn't include a space, but it keeps the space at the beginning of the line (Whisper generates these,)
• break length is configurable by the --break-lines N option. A single flag made sense to me, but if you'd rather have --break-lines and --line-length, I'm open to that.

[nogarcia]

Done! Leading and ending spaces are now stripped from the segments, and line breaking is applied to both VTT and SRT outputs. To avoid duplicating some code, I moved the leading space removal and the line breaking into a method called process_segment, which is called in both write_vtt and write_srt.