Feature request: Already-done check

[pjz]

I think xc would be more useful if you could specify an 'is this already done?' kind of check. Call it 'when'. So:

### build
Build the xc binary
When:
```
for f in cmd/xc/* ; do
  if [ $f -nt ./xc ]; then
    exit 0
  fi
exit 1
```
Requires: tests
```
go build ./cmd/xc
```

So that the requires and build blocks will only execute if the 'when' block returns true (aka exit 0). The internal default when block could be /bin/true or the like. This feature would allow for more efficient task execution, as detection of already-executed steps would be possible.

It could be spelled either when as above, or if you'd rather check for the negative, unless might work better. Or perhaps support both, for best readability.

[joerdav]

Thanks for taking the time to suggest ways to improve xc! For me I think this is a step towards the complexity I wanted to avoid with xc. I'm hesitant to add new keyword if there is a way that we can suggest a different approach that achieves the same outcome. Are you able to add the check in the script block instead?

[pjz]

Well, bash is the ultimate fallback, so of course it can be done in bash, but it seems unsightly to add guard code that conflates when something should be one with how to do it. I guess it depends on if you're aiming to be more executable documentation or more well documented task runner. For now I guess I stick with go-task and Taskfiles (though I must admit I'd prefer to write Markdown over yaml :) )