Send --help output to stdout

[codesome]

Given there are so many flags in the Loki executable, it is easier to find the flag using grep. Currently, the help text goes to stderr which makes it not possible. Do we want to consider moving it to stdout instead?

[daixiang0]

From my knowledge, help info often in stderr, why need this move?
use --help 2&>1 | grep xx is fine.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[pstibrany]

I'd like this too. Sent PR.