Implement CaptainHook

[DigitalTimK]

This PR will

• implement CaptainHook
• update documentation regarding captain hook
• close issue Introduce CaptainHook (automated checks prior commit) #243
• introduce Custom Code Fixer (for PHP CS-Fixer) to prevent change of strlen by mb_strlen in the context of Content-length (see issue Use strlen instead of mb_strlen #118)

[GhaziTriki]

It looks like we will have to tackle this down once for all.
Can we get some inspiration from this repository https://github.com/kubawerlos/php-cs-fixer-custom-fixers and create a special custom fixer for this line?

It should be doable with something like this in the custom fixer class.

public function fix(\SplFileInfo $file, Tokens $tokens)
{
    // Replace the line 'Content-length: ' . strlen($payload), with 'Content-length: ' . mb_strlen($payload),
    foreach ($tokens as $index => $token) {
        if ($token->isGivenKind(T_CONSTANT_ENCAPSED_STRING) && $token->getContent() === "'Content-length: ' . strlen(\$payload),") {
            $tokens[$index] = new Token([T_CONSTANT_ENCAPSED_STRING, "'Content-length: ' . mb_strlen(\$payload),"]);
        }
    }
}

[DigitalTimK]

done