added some conditions control

[jhuss]

• added parameter $overwrite to control(ignore/force) overwrite existing css files
• ignore if .css file exist (except if $overwrite is set)
• ignore as standalone files that start with _
• readme updated

$overwrite can be controled by debug/develop option on CMS/framework/etc...

e.g. on boltCMS, I use debug option to enable overwrite

SassCompiler::run(
    "scss/",
    "css/",
    "scss_formatter_compressed",
    ($app['config']->get('general/debug') ? True : False) // overwrite css only on "debug: true"
);