Crossterm backend not properly flushing after print

[MichaelMackus]

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).