Avoid crashes for files with position information following the timestamp

[lucas-deangelis]

Hi,
First of all, thank you for this library, I've used it to create a small tool to extract text from subtitles and it worked great. However, I've had a few crashes on files with position information following the timestamp. Here's a minimal example:

1
00:00:07,001 --> 00:00:09,015 position:50,00%,middle align:middle size:80,00% line:84,67%
This is a subtitle text

I've fixed this by modifying a bit the parsing code of the timestamp line of Subtitle.parse to be able to handle that, and added a test case. I've also edited the documentation to reflect the changes.

[GavalasDev]

Hello and thanks for the very well-thought-out pull request!

When making this library I made the (very naive) assumption that all .srt subtitle files follow a very standard and simple structure. In hindsight, bugs like this were bound to eventually appear, and I'm thankful that people like you are willing to invest their time and effort to squash them.

Greater thought should probably be put into how we handle these "weirder" cases in the future (e.g. adding a new field in Subtitle structs to contain the position information) although your temporary fix is perfect for now.

Thanks again for the great pull request, I'll be merging it soon.