nwn_script_comp: Allow taking input from stdin

[mtijanic]

Simplifies (and speeds up) hooking custom build steps such as a preprocessor.

Also would help if ncs can be emitted to stdout for a custom postprocessing step, but I figure that's particularly niche.

[ELadner]

Standard Unix command-liney things would allow a dash "-" argument to indicate hooking the input from stdin.

An easy way would be to allow the "-" argument to "-c" and pass on the stdin handle instead of opening the file.

echo "void main() { int i = 1; }" | nwn_script_comp --etc --etc -c -