Composer path updatable

Possibility to use a specific composer.phar if they are no global.
something like that
```
public function __construct()
{
  ...
  $this->composer = new Composer();
  $this->runner = new Runner($this->composer);
}

public function run($filename, array $arguments, RunConfiguration $configuration, $cliExecutor)
{
   $this->composer->setSearchPath($configuration->getComposerExecutable());
}
```