[feature request] add a mechanism to get a fixed number of rows as formatted content

[ethanpailes]

Currently, you can very easily get the contents of the current screen to re-draw with the contents_formatted method, but I'm working on a tool where I would like to be able to render the current screen plus some extra scrollback contents. I tried doing this by re-sizing the screen, but it didn't seem to work as I had hoped. I found that by patching the vt100 crate to have a new method to render formatted contents with a different selector for the rows, I could get the results that I want.

I've posted the patch to my fork and am using it in my tool. Is this something you'd be willing to upstream?