Crossterm backend not properly flushing after print

The crossterm backend doesn't appear to be properly flushing after calls to `print`. The issue is due to partial updates not flushing the stdout output stream. The bug occurs linux, perhaps it doesn't apply to all systems.

I've submitted a PR with a fix. You can also workaround the bug by calling `stdout().flush().expect("Command fail");` in your tick function (although that will only flush the previous frame's buffer I believe, since the actual output happens after the call to tick).