Relaxing crossterm dependency version

[svartalf]

This is a follow-up to #153.

Since crossterm 0.9 version development is still in progress (as an example, see this bug), forcing to use one specific version is a little bit excessive, so this PR relaxes crossterm version to ^0.9.
@TimonPost confirmed that crossterm adheres to semantic versioning, therefore version can be relaxed safely.

As an addition, I removed the crossterm_backend feature because of two reasons:

1. crossterm feature is exists already (because crossterm as a depedency is optional)
2. Code that introduces it was merged into master 6 hours ago (at the time of creating this PR) and was not published yet, therefore possibility of breaking something is really small.

P.S. I have personal request also - can there be new version of tui published after merge of this PR? This is a only thing which blocks me.

[fdehau]

Merged =). I was about to revert the crossterm_backend feature myself so that made perfect sense to me. I've also published a new version (v0.6.0).