Allow users to provide replacement functions for standard I/O

[bbassett-tibco]

To allow users to integrate the SBDF code with systems that do not utilize C FILE * pointers and the C standard I/O functions (from <stdio.h>), such as Python io.BytesIO objects for in-memory buffers, provide a mechanism for using alternate I/O functions. This would be useful for implementing spotfiresoftware/spotfire-python#63.