Support for passing #defines as command-line parameters

[nvining-work]

We would like to be able to have an equivalent of fxc's -D parameter, which lets a #define be created via a command-line parameter. (Currently of course -D is being used to emit HLSL; so... I don't know what the parameter should be.) Unless I'm missing something, this does not seem to be implemented in glslangValidator.

[johnkslang]

Right, this is missing functionality. It is also a duplicate of #87. I will close this one, and list it in #362. Good to be aware of addition need here.