Make xtrace optional

[chrisalbright]

xtrace output is useful in some cases, but in general would be better if not enabled. This is on by default and it seems there is no way to disable it for xc. A -x or -v flag would be useful for enabling this output when desired.

[joerdav]

Thanks for looking at xc, I would be open to creating a flag to disable this. But I think this is a common feature of other task runners I've seen, to log out what's about to happen.

But open to be persuaded otherwise if there's a compelling case to have it off by default.

[chrisalbright]

Unless I'm trying to work out something that's going wrong I find the verbose output to be disruptive, personally. I've been tinkering with it for a while. I'll open a PR shortly and we can see what you think. I started with a global flag, but after thinking a bit it seems like something worth setting at the task level. Maybe with a global disable?

Once I have the PR open, we can see what works best.