lib/mp-readline: Add emacs-style control characters for cursor movement.

Disabled by default.  Adds 108 bytes to Thumb2 arch when enabled.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 9f0e909..a345e5f 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -303,6 +303,11 @@
 #define MICROPY_HELPER_REPL (0)
 #endif
 
+// Whether to include emacs-style readline behavior in REPL
+#ifndef MICROPY_REPL_EMACS_KEYS
+#define MICROPY_REPL_EMACS_KEYS (1)
+#endif
+
 // Whether port requires event-driven REPL functions
 #ifndef MICROPY_REPL_EVENT_DRIVEN
 #define MICROPY_REPL_EVENT_DRIVEN (0)