Use the context provide to UsageWithContext public function

[GustavoCaso]

Currently, at work, we use gopsutil. We wanted to use the disk.UsageWithContext to allow setting some timeout when calling the Usage function.

We realized that the function UsageWithContext does not use the ctx provided.

It think would be a good addition to this great package.

That way, the user can do:

ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
defer cancel()

usage, err := UsageWithContext(ctx, "/")
if err != nil {
	log.Fatal(err)
}

Before adding a test or applying this pattern to other functions *WithContext, I wanted to know if we would be okay with having it in the package. If so, I can add tests and extend the pattern to the rest of the *WithContext functions.

Looking forward for your feedback.