487.future task triggers

As discussed on cylc-dev in response to @trwhitcomb's post.

```
graph = "foo[T+24] => bar"
```

I guess this looks impossible from a traditional fixed-cycling scheduler perspective, but in the cylc way of doing things I can't see anything wrong with it.  And it clearly provides a cleaner way of defining suites with multi-cycle cold-starts (see the cylc-dev discussion).

Also, this is a small change that does not affect existing functionality.