This PR was merged into the 5.2-dev branch.

Discussion
----------

[Console] Add support for true colors

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #26576, Fix #19844 <!-- prefix each issue number with "Fix #", if any -->
| License       | MIT
| Doc PR        | -

This PR adds support for true colors in the Console component.

Instead of adding many ways to add more colors than the current "default" ones, I've opted to only add true color support via Hex CSS colors. If you have something else (RGB, HSV, ...), you need to first convert it to a CSS color. I've also decided to not support the ANSI 256 colors as most terminals support true colors nowadays.

If true colors are not supported by the terminal, we fall back to the "nearest" default color.

`<fg=green;bg=blue>` is now equivalent to `<fg=#00ff00;bg=#00f>`.

The `Color` class is usable outside of the Console framework as well:

```php
$color = new Color('black', 'white');
echo $color->apply("foo");
echo "\n";

$color = new Color('red', 'yellow');
echo $color->apply("foo");
echo "\n";

$color = new Color('#000000', '#ffffff');
echo $color->apply("foo");
$color = new Color('#000', '#fff', ['underscore', 'reverse']);
echo $color->apply("bar");
echo "\n";
```

Rainbow time!

```php
function rainbowColor($i) {
    $h = (int) ($i / 43);
    $f = (int) ($i - 43 * $h);
    $t = (int) ($f * 255 / 43);
    $q = 255 - $t;

    if ($h == 0) {
        return new Color('', sprintf('#FF%02x00', $t));
    } elseif ($h == 1) {
        return new Color('', sprintf('#%02xFF00', $q));
    } elseif ($h == 2) {
        return new Color('', sprintf('#00FF%02x', $t));
    } elseif ($h == 3) {
        return new Color('', sprintf('#00%02xFF', $q));
    } elseif ($h == 4) {
        return new Color('', sprintf('#%02x00FF', $t));
    } elseif ($h == 5) {
        return new Color('', sprintf('#FF00%02x', $q));
    }
}

for ($i = 0; $i < 128; $i++) {
    echo rainbowColor($i)->apply(' ');
}
echo "\n";

for ($i = 255; $i >= 128; $i--) {
    echo rainbowColor($i)->apply(' ');
}
echo "\n";
```

![image](https://user-images.githubusercontent.com/47313/81796170-59af5e00-950d-11ea-8203-18c13ae8a07e.png)

Commits
-------

d066514 [Console] Add support for true colors