Add an example program for simple C++ API usage

The program demonstrates basic shader compilation using the Shaderc
C++ API.  For clarity, each method is deliberately self-contained.

Techniques demonstrated:
- Preprocessing GLSL source text
- Compiling a shader to SPIR-V assembly text
- Compliing a shader to a SPIR-V binary module
- Setting basic options: setting a preprocessor symbol.
- Checking compilation status and extracting an error message.