mpy-cross: Add --version command line option to print version info.

Prints something like:

MicroPython v1.10-304-g8031b7a25 on 2019-05-02; mpy-cross emitting mpy v4
diff --git a/py/persistentcode.h b/py/persistentcode.h
index b27c3de..2074c64 100644
--- a/py/persistentcode.h
+++ b/py/persistentcode.h
@@ -30,6 +30,9 @@
 #include "py/reader.h"
 #include "py/emitglue.h"
 
+// The current version of .mpy files
+#define MPY_VERSION 4
+
 enum {
     MP_NATIVE_ARCH_NONE = 0,
     MP_NATIVE_ARCH_X86,