Allow all function params to be checked by 'unused'

This allows passing a value other than true/false to the 'unused' option to allow for checking all function params.  For
example:

    function foo(a, b) {
		return b;
	}

If 'unused' === true, no warnings will be thrown, since 'b' is used.  If 'unused' === 'all', then a warning will be
thrown, since 'a' is not used.

References:
* Makes GH-607 the default, but optional, rather than mandatory
* Closes GH-778