Add hello world example for engine.

research/_examples/engine_hello_world.c

+/*
+This example demonstrates how to initialize an audio engine and play a sound.
+
+This will play the sound specified on the command line.
+*/
+#define MINIAUDIO_IMPLEMENTATION
+#include "../../miniaudio.h"
+#include "../miniaudio_engine.h"
+
+int main(int argc, char** argv)
+{
+    ma_result result;
+    ma_engine engine;
+
+    if (argc < 2) {
+        printf("No input file.");
+        return -1;
+    }
+
+    result = ma_engine_init(NULL, &engine);
+    if (result != MA_SUCCESS) {
+        printf("Failed to initialize audio engine.");
+        return -1;
+    }
+
+    ma_engine_play_sound(&engine, argv[1], NULL);
+
+    printf("Press Enter to quit...");
+    getchar();
+
+    ma_engine_uninit(&engine);
+
+    return 0;
+}