ref #16, #22 : add "offset" argument

Allows to start processing the input audio at some offset from the
beginning. Useful for splitting a long job into multiple tasks.

whisper.h

@@ -102,6 +102,7 @@ extern "C" {
         enum whisper_decode_strategy strategy;
 
         int n_threads;
+        int offset_ms;
 
         bool translate;
         bool print_special_tokens;