Function capture 

pub fn capture<F: FnOnce()>(mode: OutputMode, f: F) -> String

Capture output from a closure in the specified mode.

Example

use fastapi_output::prelude::*;
use fastapi_output::testing::*;

let output = capture(OutputMode::Plain, || {
    let out = RichOutput::plain();
    out.success("Hello");
});

assert_contains(&output, "Hello");