Colorized flexible tables

# Example

```rust
// Import Veg, ColoredString, and Colorize from veg::colored
use veg::colored::{ColoredString, Colorize, Veg};

// Create a custom type
struct Point {
    x: f32,
    y: f32,
}

// Implement a method that creates a Box of the custom type
impl Point {
    fn new(x: f32, y: f32) -> Box<Point> {
        Box::new(Point { x, y })
    }
}

// Implement the veg::Table::row method to define how to print the custom type
impl veg::colored::Table for Point {
    fn row(&self) -> Vec<ColoredString> {
        /*
        // Just color both red
        [self.x, self.y]
            .iter()
            .map(|x| x.to_string().red())
            .collect()
        */

        /*
        // Color x green and y red
        vec![self.x.to_string().green(), self.y.to_string().red()]
        */

        // Color negative red, 0 plain, and positive green
        [self.x, self.y]
            .iter()
            .map(|&x| {
                let s = format!("{x:.1}");
                if x < 0.0 {
                    s.red()
                } else if x > 0.0 {
                    s.green()
                } else {
                    ColoredString::from(s)
                }
            })
            .collect()

        // ...
    }
}

// Create a Veg via the table method with a header definition
let mut v = Veg::table(&format!("{}|{}\n-:|-:", "x".cyan(), "y".magenta()));

// Add a single point
v.push(Point::new(-2.0, 4.0));

// Add a bunch of points
v.append(&mut vec![
    Point::new(-1.5, 3.0),
    Point::new(-1.0, 2.0),
    Point::new(-0.5, 1.0),
    Point::new(0.0, 0.0),
    Point::new(0.5, -1.0),
    Point::new(1.0, -2.0),
    Point::new(1.5, -3.0),
    Point::new(2.0, -4.0),
]);

// Render as a markdown table
let m = v.markdown().unwrap();
/*
// NOTE: This assert doesn't pass in a doctest... please see `/tests/colored.rs`
// and/or run `cargo test --features colored --test colored -- --nocapture`.
assert_eq!(
    m,
    ColoredString::from(
        "\
| \u{1b}[36m   x\u{1b}[0m | \u{1b}[35m   y\u{1b}[0m |
|-----:|-----:|
| \u{1b}[31m-2.0\u{1b}[0m | \u{1b}[32m 4.0\u{1b}[0m |
| \u{1b}[31m-1.5\u{1b}[0m | \u{1b}[32m 3.0\u{1b}[0m |
| \u{1b}[31m-1.0\u{1b}[0m | \u{1b}[32m 2.0\u{1b}[0m |
| \u{1b}[31m-0.5\u{1b}[0m | \u{1b}[32m 1.0\u{1b}[0m |
|  0.0 |  0.0 |
| \u{1b}[32m 0.5\u{1b}[0m | \u{1b}[31m-1.0\u{1b}[0m |
| \u{1b}[32m 1.0\u{1b}[0m | \u{1b}[31m-2.0\u{1b}[0m |
| \u{1b}[32m 1.5\u{1b}[0m | \u{1b}[31m-3.0\u{1b}[0m |
| \u{1b}[32m 2.0\u{1b}[0m | \u{1b}[31m-4.0\u{1b}[0m |
\
        ",
    ),
);
println!("{m}");
*/
```