question: living Cursive seems to locks std out

Hi @gyscos

I'm new to Rust and I'm inspired by your work on Cursive :+1:
Can you help me understand why any new Cursice instance is blocking stdout ?

Here is two cases I compared :

```rust
extern crate cursive;

use cursive::Cursive;
use cursive::views::{Dialog, TextView};

fn main() {
println!("A");

let mut siv = Cursive::new();

println!("B");

siv.add_layer(
Dialog::around(TextView::new("Hello Dialog!"))
.title("Cursive")
.button("Quit", |s| s.quit()),
);

println!("C");
}

# $ cargo run
# A
```

Compared to

```rust
extern crate cursive;

use cursive::Cursive;
use cursive::views::{Dialog, TextView};

fn main() {
println!("A");

{
let mut siv = Cursive::new();

println!("B");

siv.add_layer(
Dialog::around(TextView::new("Hello Dialog!"))
.title("Cursive")
.button("Quit", |s| s.quit()),
);
}

println!("C");
}

# $ cargo run
# A
# C
```

Apparently as long as a Cursive instance is alive, we can't print to stdout.
Do I get it right ?

Thanks