Enable the YouTube video checker by default on new installations.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@975648 b8457f37-d9ea-0310-8a92-e5e31aec5664
Minor: Fix whitespace.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@975649 b8457f37-d9ea-0310-8a92-e5e31aec5664