placeholder check should be smarter

[wojciechczerniak]

@extend has more advanced syntax than currently supported. With extending multiple selectors and optional extending the line can look like:

@extend .design !optional, $specialDesign !optional

and we should catch placeholder preferences not only at the beginning of the line.

Also should support multiple occurrences #266

@extend .firstPlaceholder, $firstPlaceholder, .anotherSelector, $anotherPlaceholder

[rossPatton]

it's a little smarter now, gonna close this and just reference #269 since that's the real issue here