ShaderLab boolean parameters to support uber-shaders

It will be very convinient to create special type of ShaderLab parameters as boolean. You can make this parameter editable only from inspector and accessible only for read from scripts, so all shader permutations are ready before package build phase and we don't need to generate all combinations as for 'keywords'. This boolean should generate #define with the same name if 'true' and do nothing in other case.