Make --filename only apply when stdin is used
Make --filename only apply when stdin is used

And instead introduce a positional argument for the path to the file
when not using stdin.

I'll follow up by renaming --filename to --stdin-file-path to make the
different usecases clearer.

trotzig/import-js,trotzig/import-js,Galooshi/import-js,trotzig/import-js