allow phpmetrics --git option

[flavius]

phpmetrics is able to show some basic git statistics, if instructed so. Add option in phpqa to make it possible.

[zdenekdrahos]

Please add new option to:

• documentation
• .phpqa.yml

Refactoring:

• if conditions ?
• maybe phpmetrics.gitBinary would be better name ?

[zdenekdrahos]

If conditions are not really understandable without looking into vendor/bin/phpmetrics --help. I would prefer something like that:

$gitBinary = $this->config->value('phpmetrics.git');
if ($gitBinary) {
$args['git'] = is_file($gitBinary) ? $gitBinary : 'git';
}