--debug option can optionally be Boolean. Fixes GH-983.

Fixes #983 where `grunt --verbose --debug` works but `grunt --debug --verbose` does not. This is because `--debug` has the only type `Number` and interprets `--verbose` as the given number. In which, of course, is an invalid number so both options are canceled.

`grunt --debug=1 --verbose` does work as expected though thus illustrating the issue.

This PR allows `--debug` to be optionally set as a boolean and fixes the issue.

Tests included and adds a necessary `noop` internal task to speed things up.