Add Screen::all_contents_formatted

[austinjones]

I'm thinking of using vt100 in tab-rs, to efficiently render the scrollback buffer. But I need the entire stored contents, including primary and alternate buffers, so that once scrollback has been completed, interaction with the user can proceed with live stdout.

This PR adds all_contents_formatted to Screen, which:

• Fills both the primary and alternate buffer, leaving the currently active buffer visible
• Returns content for all lines in scrollback, not just visible lines

[chris-laplante]

I would also like a way of getting the contents of the scrollback buffer without having to mess with scrollback position. Unfortunately, looking at the age of this issue I guess the repo is dead?

[ethanpailes]

I wrote ethanpailes@bcc2e63 which does something similar but accepts a parameter to determine the number of rows to include in the contents buffer. That's a bit more flexible since you can always just fetch the size and set n=rows.