read from `stdin` if argc==0

[Ma27]

Several CLI tools simply read from stdin if no file is given by default.
This can be helpful when piping several processes together. Currently you'd have to use ugly bash-isms to achieve this.

With such a change, pipes like this will be possible:

# replace chars in a script before running
cat foo.bf | sed -e "magic here" | interfuck

# execute a script from a remote location (be careful with this!)
curl http://example.com/example.bf | interfuck

[TrueDoctor]

#12 would be really useful for testing purposes