Allow to use glslmin with the system() function

It is currently not possible to use glslmin with the system() function
(or anything else which does not have a tty) without using a pipe
(`cat file | glslmin`), since it will never map the arguments to a
stream if the input is not a tty.

This commit fixes this issue by always mapping the arguments to
streams, and fallback on stdin if there is no argument (the previous
behavior was to display the help message).
Merge pull request #1 from arcanis/patch-1

Allow to use glslmin with the system() function