Added --vn option to generate a C header file containing a variable assigned...

Added --vn option to generate a C header file containing a variable assigned to the hex representation of the shader. This is a standard feature on Microsoft's HLSL compiler and it allows developers to include pre-compiled shaders directly into the code. This option enables "Hex output", so it is NOT required to specify -x as well. The output file name is preserved, so no ".h" extension is added. If you want the output file to have ".h" extension then you have to specify it on the output file name. The generated header file uses the "#pragma once" pragma to avoid multiple inclusions.