compiler/commands: Always write usage to stdout
compiler/commands: Always write usage to stdout

Usage is compiler command output, not diagnostics, so we shall output it to
stdout. Also this makes: nim -h | less or nim -h | grep ... working.

jsanjuas/Nim,nanoant/Nim,singularperturbation/Nim,dom96/Nim,mbaulch/Nim,jsanjuas/Nim,zachaysan/Nim,tulayang/Nim,singularperturbation/Nim,singularperturbation/Nim,mbaulch/Nim,sarvex/Nim-lang,russpowers/Nim,endragor/Nim,msmith491/Nim,nanoant/Nim,jsanjuas/Nim,dom96/Nim,jsanjuas/Nim,tulayang/Nim,dom96/Nim,haiodo/Nim,endragor/Nim,endragor/Nim,sarvex/Nim-lang,jsanjuas/Nim,endragor/Nim,tulayang/Nim,jfhg/Nim,zachaysan/Nim,jfhg/Nim,zachaysan/Nim,nanoant/Nim,russpowers/Nim,haiodo/Nim,jsanjuas/Nim,dom96/Nim,Matt14916/Nim,jfhg/Nim,Matt14916/Nim,jfhg/Nim,sarvex/Nim-lang,zachaysan/Nim,Matt14916/Nim,dom96/Nim,sarvex/Nim-lang,haiodo/Nim,tulayang/Nim,dom96/Nim,haiodo/Nim,jfhg/Nim,russpowers/Nim,singularperturbation/Nim,haiodo/Nim,nanoant/Nim,tulayang/Nim,msmith491/Nim,mbaulch/Nim,sarvex/Nim-lang,russpowers/Nim,haiodo/Nim,zachaysan/Nim,nanoant/Nim,Matt14916/Nim,nanoant/Nim,russpowers/Nim,russpowers/Nim,dom96/Nim,mbaulch/Nim,jfhg/Nim,msmith491/Nim,sarvex/Nim-lang,Matt14916/Nim,Matt14916/Nim,singularperturbation/Nim,Matt14916/Nim,nanoant/Nim,jsanjuas/Nim,msmith491/Nim,jfhg/Nim,mbaulch/Nim,zachaysan/Nim,endragor/Nim,tulayang/Nim,msmith491/Nim,singularperturbation/Nim,endragor/Nim,russpowers/Nim,singularperturbation/Nim,zachaysan/Nim,mbaulch/Nim,mbaulch/Nim,endragor/Nim,sarvex/Nim-lang,msmith491/Nim,haiodo/Nim,msmith491/Nim