Support parsing srt without sequence numbers.

[samuel]

There are many srt in the wild that are missing sequence numbers. It's easy to detect by looking for an empty line vs index/sequence in the line before the time. In my analysis of a good number of srt I have not seen ones missing the empty line, but it's feasible that these also exist. This shouldn't affect parsing otherwise, however, and in my experience the missing sequence number is common enough that it's useful to handle.

[samuel]

Examples of these srt files: https://media.defcon.org/DEF%20CON%2023/DEF%20CON%2023%20video/

[asticode]

Thanks for the PR! ❤️

[asticode]

Let me know whether you need a tag 👍