Allow to get other metrics

[emibcn]

Allow the user to get a different metric than just elements. For example:

• statements
• conditionals
• methods
• etc.

Maybe adding a field for selecting that and then using it at PHP like:

$metricName = $argv[3] || 'elements';

# (...)

foreach ($metrics as $metric) {
    $totalElements += (int) $metric[$metricName];
    $checkedElements += (int) $metric['covered' . $metricName];
}

[emibcn]

Changed code to merge with current version.

Adds:

• An input in action.yml
• An optional 5th argument for the script which defaults to elements
• An optional 2nd argument for fromXml which defaults to elements
• Documentation in README
• Some test