--input-source-map not respected

Situation:
I produce a bundle with another tool that has a source map to the original files. I'd like to minify this bundle with babili.

I've run this command (and variations with `--inputSourceMap` etc)
`babili --source-maps --input-source-map=./dist/bundle.js.map -o ./dist/bundle.min.js ./dist/bundle.js`

Expected: `./dist/bundle.min.js.map` maps lines to the original files
Actual: `./dist/bundle.min.js.map` maps lines to `./dist/bundle.js`