`add_lines()`: allow specifying desired number of lines or minutes

[mdingemanse]

Suggestion from @bvreede:

Would it be an option to change the arguments so that a user can indicate (1) the total length (in minutes) of the fragment they want to visualize, and (2) the number of lines?

Then these values (extract_length() and window_size()) can be calculated, and the user has easier numbers to enter.

An optional argument could also be the timestamp where the visualization should start.

_Originally posted by @bvreede in https://github.com/elpaco-escience/talkr/pull/10#discussion_r1159651156_

[mdingemanse]

Reposting my comments from that pull request thread before we lose track of them:

ooh I like this. Like, give me this full convo in 10 lines please; or, give me this convo in lines of 5 minutes. Yes that makes a lot of sense. The two variables n_lines and line_duration would be mutually exclusive then?

Thinking ahead of plotting, would it make sense to have the line duration of n_lines be some nearest reasonable round number?

BTW I also like the humanness of working with whole minutes, not milliseconds, even if under the hood we deal in ms. Still, from working with transcripts I do anticipate that sub-minute lines are common (e.g., 10-20 seconds), so perhaps line_duration_seconds (seconds) and line_duration_minutes? Or line_duration=10,unit="minutes" with a sensible default? Trick is to keep the number of options managable and the defaults useful.