Accept Custom Formatter Classes

This pull request allows one to specify the formatter class, which is used by Athletic, via command line.

If a class name is specified as `formatter` argument, then this class is used to format the benchmark results:
```bash
$ php ./vendor/bin/athletic --formatter "My\Custom\FormatterClass" --path /home/ProjectDir/benchmarks/ --bootstrap /home/ProjectDir/vendor/autoload.php 
```
Classes that are used as formatter must implement `Athletic\Formatters\FormatterInterface` and provide a constructor without parameters.

The previous behavior of the `formatter` argument is preserved, which means that it is still possible to pass short names for built-in formatters.