Move timestamps to CSS div:before statements

[dltj]

Addresses #129

If @seconddayout is like me, the issue is with the YouTube video transcripts. Although it looks like one can select across <div>s...

...when Hypothesis grabs the text, it only gets the part in the first <div>:

I ran into this same sort of problem when I was creating an equivalent to droppdf for podcasts (see unchecked-transcript). To solve the problem, I put the timestamp into a CSS :before declaration. See this example for what it looks like in practice—notably the first Hypothesis annotation that crosses three <div>s.

This pull request is my best guess at how this solution would work with droppdf. Unfortunately, I couldn't get all of the droppdf prerequisites to install on my machine, so I wasn't able to do an end-to-end test of the code changes.