Add INI_CUSTOM_ALLOCATOR to allow using a custom memory allocator. Per the README:
By default when using the heap, the standard library's
malloc,free, andreallocfunctions are used; to use a custom allocator, specify-DINI_CUSTOM_ALLOCATOR=1(and-DINI_USE_STACK=0). You must define and link functions namedini_malloc,ini_free, and (ifINI_ALLOW_REALLOCis set)ini_realloc, which must have the same signatures as thestdlib.hmemory allocation functions.
See tests/unittest_alloc.c for an example.
Fixes #118.