Add ability to skip main query

It'd be nice to have a `skip_main_query` option to enable not running the main query at all for pages which are just static, or redirects.

Andy Skelton's answer from [wp-hackers](https://groups.google.com/forum/#!topic/wp-hackers/fsgDeZe6sx0) is a good start, but `WP::query_posts` can be overridden in a nicer way than when that answer was added.