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, and realloc functions are used; to use a custom allocator, specify -DINI_CUSTOM_ALLOCATOR=1 (and -DINI_USE_STACK=0). You must define and link functions named ini_malloc, ini_free, and (if INI_ALLOW_REALLOC is set) ini_realloc, which must have the same signatures as the stdlib.h memory allocation functions.

See tests/unittest_alloc.c for an example.

Fixes #118.