Create a dense unicode-based renderer

[georgmu]

When comparing to the "qrencode" command line tool, qrcode-rust's char renderer lacks a dense output mode where one console line prints two qrcode lines.

I have implemented a solution using your library. Maybe you would like to add it.

Here is a gist with my implementation:

https://gist.github.com/georgmu/eb37863f0bea3b8a1d18670587553e8a

[cab404]

https://github.com/kennytm/qrcode-rust/pull/42