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 (crossterm-rs/crossterm#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

[ynqa]

@cih9088 First of all, thanks for your interests in this project and contributions 🎉

I believe I understand what you're trying to achieve. On the other hands, I would prefer not to attach anything to its arguments as much as possible (prompt() is similar to build method in the builder pattern).

Please follow the approach of adding fields to manage io::Write for each preset, along with methods to modify them, just like with other fields before that.

[cih9088]

@ynqa

Changed to follow builder pattern.
Please feel free to change the naming of setter.