Allow a default STLor gcode file to be passed in via the command line.

This is so 3rd party design applications can output an STL file and then
launch Repetier-Host with the STL file as input.

P.S.  This is my first code contribution for Repetier-Host so sorry if I did anything wrong.  I noticed some code was removed for what I think was command line options so I'm glad to change to the code or discuss what the need is.

-burhop (mark.burhop@gmail.com)