Rename arguments: `file` to `output-file` and `expression-file` to `file`.

If I could, I would have renamed `expression-file` to `file` and renamed the current `file` argument to `output-file`. The naming of the arguments is inaccurate and confusing.

#168 Fixes `expression-file` so it supports any given file. This means that the `expression` part is just useless now. Also, the current `file` argument doesn't imply that it is an `output` file.