Preserve @media if necessary

Fixed #1

- add preserveMediaQuery parameter allow preserve @media query in CSS
- default behavior is same as before (remove all not valuable media query)
- improve regular expression for matching valuable @media query 
- add support for `only` keyword in [media query](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries?redirectlocale=en-US&redirectslug=CSS%2FMedia_queries)
- add unit tests for this functionality