added some conditions control

* added parameter `$overwrite` to control overwrite
* ignore files that start with _
* ignore if .css file exist (except if `$overwrite` is set)

`$overwrite` can be controled by debug/develop option on CMS/framework/etc...

code example on boltCMS:
```php
SassCompiler::run(
    "scss/",
    "css/",
    "scss_formatter_compressed",
    ($app['config']->get('general/debug') ? True : False) // compile scss only on "debug: true"
);
```