Proposal: add a --trace, --trace=on|off to output trace instead of explicitly enable in each test case

[xhd2015]

Currently, if we want to visualize trace of each test ,we need to manually add a line:

func TestHelloWorld(t *testing.T){
    defer trace.Begin()()
    ...
}

But if we don't want trace, we need to manually remove that line, this causes some inconvenience.

So, propose adding a flag --trace flag, that will look like:

xgo test --trace -run TestHelloWorld

If --trace=off or not present, no trace will output.

[xhd2015]

But it seems go test -trace trace.out is already take, so we need another flag.

Change this flag to --stack-trace or --xgo-trace or --strace.

--strace is better because it is shorter.

[xhd2015]

resolved in #69