Change input/output behavior to be similar to sass (Ruby).

- When no parameters are given, the input is read from stdin and the
  output is printed to stdout.
- When 1 parameter is given, it is interpreted as input path and the
  output is printed to stdout.
- When 2 parameters are given, the first one is used as input path and
  the second one is used as output path.
- When more than 2 parameters are provided, an error message is printed.