Fix node groupnames

[clue]

This PR adds support for using task name that include group names, like this:

task('a:b:c', function () { });

This will now automatically create the nodes named a, a:b and a:b:c. So this is equivalent to the longer form:

group('a', function () {
  group('b', function () {
    task('c', function () { });
  });
});

And equivalent to any of these mixed forms:

group('a:b', function () {
  task('c', function () { });
});

or

group('a', function () {
  task('b:c', function () { });
});