new utf8proc_map_custom for hooking in user-defined custom mappings (#89)

* new utf8proc_map_custom for hooking in user-defined custom mappings

* whoops, add test program

* NEWS, version bump for 2.1

* change test functions to static so that gcc doesn't complain about missing prototypes

test/custom.c

@@ -0,0 +1,27 @@
+#include "tests.h"
+
+static int thunk_test = 1;
+
+static utf8proc_int32_t custom(utf8proc_int32_t codepoint, void *thunk)
+{
+    check(((int *) thunk) == &thunk_test, "unexpected thunk passed");
+    if (codepoint == 'a')
+        return 'b';
+    if (codepoint == 'S')
+        return 0x00df; /* ß */
+    return codepoint;
+}
+
+int main(void)
+{
+    utf8proc_uint8_t input[] = {0x41,0x61,0x53,0x62,0xef,0xbd,0x81,0x00}; /* "AaSb\uff41" */
+    utf8proc_uint8_t correct[] = {0x61,0x62,0x73,0x73,0x62,0x61,0x00}; /* "abssba" */
+    utf8proc_uint8_t *output;
+    utf8proc_map_custom(input, 0, &output, UTF8PROC_CASEFOLD | UTF8PROC_COMPOSE | UTF8PROC_COMPAT | UTF8PROC_NULLTERM,
+                        custom, &thunk_test);
+    printf("mapped \"%s\" -> \"%s\"\n", (char*)input, (char*)output);
+    check(strlen((char*) output) == 6, "incorrect output length");
+    check(!memcmp(correct, output, 7), "incorrect output data");
+    free(output);
+    return 0;
+}