Add caption input/output

[kpister]

We will want to allow captions to be input with the video (since some youtubers will have those) and also output captions in the foreign languages.
These are stored in SRT files which have the following format

<section number>
<start time> --> <end time>
<Caption text
Can be multiple lines>

Example:

1
00:00:00,000 --> 00:00:04,400
Here is the first caption of the video.

2
00:00:06,200 --> 00:01:00,000
Then we have a really long
caption that takes up most of the video.