Add ability to customize HMM_ prefix (#114)

* Add prefix macro and use it everywhere

* Add lightweight test for prefix

* Add a little doc blurb

test/HandmadeMathDifferentPrefix.cpp

@@ -0,0 +1,12 @@
+#define HMM_PREFIX(name) WOW_##name
+
+#define HANDMADE_MATH_IMPLEMENTATION
+#define HANDMADE_MATH_NO_INLINE
+#include "../HandmadeMath.h"
+
+int main() {
+    hmm_vec4 a = WOW_Vec4(1, 2, 3, 4);
+    hmm_vec4 b = WOW_Vec4(5, 6, 7, 8);
+
+    WOW_Add(a, b);
+}