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

[aminya]

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.