Add method to check if optimizers are installed

[Flynsarmy]

This PR adds a checkOptimizers() method that returns an array of all defined optimizers and whether or not they're installed. This is useful, for example, when displaying a configuration page explaining what needs to be installed for certain image types to compress correctly.

I had to make some private fields public for this to work.

Example usage:

$factory = new \ImageOptimizer\OptimizerFactory();
print_r($factory->checkOptimizers());

Array
(
    [/usr/local/bin/optipng] => true
    [/usr/local/bin/pngquant] => true
    [/usr/local/bin/pngcrush] => true
    [pngout] => false
    [/usr/local/bin/advpng] => true
    [/usr/local/bin/gifsicle] => true
    [/usr/local/bin/jpegoptim] => true
    [/usr/local/bin/jpegtran] => true
    [/usr/local/bin/svgo] => true
)