Feature request: Only enable user chosen Vulkan subset

Currently, volk creates function pointers for all Vulkan API and extensions that are defined in the vulkan header. If instead the user could define (I suppose via macro defines) which core version + extension are being used, it would have the following advantages:

• Only functions that the user plans to use are actually resolved, speeding up initialization
• It prevents the user from accidentally using functions from extensions or core versions that were not declared

Would also like to have an option to hide the global device function prototypes in case the application only allows the use of VolkDeviceTable.

VOLK_NO_DEVICE_PROTOTYPES or alike.