Testing this, I have renamed the Elm entry file to Main.elm, and used this commandline:
elm-live Main.elm -- --output=elm.js
With this setup, elm-live responds with a 404 Not Found with a the following body:
File Not Found
I see that the elm.js file was created tough.
Based on the guide from @dmy at Elm reactor - init with flags for 0.19 - #10 by dmy, I found this working setup:
elm-live Main.elm -d outputtoignore -- --output=outputtoignore/index.html