be more strict with default checker settings

If a filetype has default checker settings, then only allow those
checkers to be used by default. The user must manually specify other
checkers to use if they dont want the default.

In future the `s:defaultCheckers` hash should be more complex (or
possibly an object) that allows us to specify "only" vs "preferred".

This change was made in response to the html filetype using the w3
checker when tidy is not installed.