Auto specialization constant IDs

Heya, I recently found the functions `SetAutoBindUniforms()` and `SetBindingBase()` and I must admit they have been very useful to me. However, I also need such functionality for dealing with specialization constants and there is not one implemented, which is why I decided to write this small issue.

What I propose is adding three more methods - `SetAutoMapConstants(bool)` (which will search for the following syntax: `layout() const Type varName = someValue;` and will automatically set the `constant_id` property for each field), `SetConstantsBase(uint)` (this one will set the starting `constant_id`, so, supposing we use `SetConstantsBase(10)`, `constant_id`s will start from 10 and grow as more fields are present) and `SetPreserveConstants(bool)` (does the same thing as `SetPreserveBindings()` - will either remove all specified `constant_id`s, or not).

It is really just extending the logic of `shaderc::CompileOptions` to not only work with locations and bindings, but also provide support for specialization constants.