Add custom writer

[cih9088]

Rationale

Give users a bit of freedom to choose std::io::Write. One specific use case is to pipe stdout and keep promkit interactive by rendering promkit to stderr.

Limitation

One caveat is that piping stdout does not work for now because cross term::cursor::position() uses stdout by default (https://github.com/crossterm-rs/crossterm/issues/919).

Although it is not possible to pipe stdout due to the limitation of crossterm for the moment (hopefully fixed in near future), I believe this would be useful.

For custom use, I just added termion to get cursor position