Cannot pass multiple input files

[bdukes]

With the main elm executable, I can run elm make src/Main.elm src/Main2.elm --output dist/out.js and it'll compile both modules. However, elm-optimize-level-2 src/Main.elm src/Main2.elm --output dist/out.js will only compile the first (in fact, compilation errors in the second module will be ignored altogether). Is this feature intended to be supported?