Custom media types!

• Added the ability to define custom media types for individual mq statements.
• BREAKING CHANGE: By default, no media type is added to the media query (previously it added "screen" as the media type to all media queries).
  • If upgrading from v1.x you may want to add "screen" to the end of all your MQ statements to retain consistency with v1.x.
  • eg. from @include mq(max, 800px) to @include mq(max, 800px, 'screen').