get terminal size with alternate file descriptor (e.g. stderr)

Motivation: I discovered this project via the lovely terminal application broot. One of the biggest problems I have is difficulty capturing the selection like fzf allows: selection="$(fzf)". Instead, the broot dev recommends this ugly workaround. I forked broot to use stderr everywhere, and everything works just as before. However, when I try to capture selection, it panics on get_terminal_size since stdout is no longer a terminal.

Proposed solution: Since #89 is already proposing an API change on get_terminal_size, I think instead we should leave get_terminal_size alone, and add a new fn like get_filedesc_size with both this and #89's changes. This would then only be a minor version bump in semver.