Support negative log filter syntax (starts with an exclamation mark)

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Revert "Comment one unsupported feature"

This reverts commit 73b6d23c832d7cdd2296eed5a14ecc024e54f945.

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Merge branch 'log-filter'