API CHANGE: Add support for custom memory allocators.

This commit changes the following APIs to add an additional parameter
called pAllocationCallbacks. This is a pointer to a
ma_allocation_callbacks structure which is used to perform the actual
allocation/free.

Allocation callbacks can be set in ma_context_config for contexts and
ma_decoder_config for decoders. If it's left unset it will use
MA_MALLOC(), MA_REALLOC() and MA_FREE().