Add a C shell, which currently just passes through

This is a first step toward moving the UI to C and
adding file and network I/O in C. The fractal core
will remain in assembler as well as the multiplier.

mandel.c

@@ -0,0 +1,15 @@
+/**
+ * The UI and I/O wrapper for the Mandelbrot runner, in C.
+ *
+ * For the moment *all* logic is in mandel-core.s, I'm just
+ * trying to get this to run within a cc65 environment.
+ * Eventually just the inner loop fun will live in there.
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include "mandel.h"
+
+void main(void) {
+    mandel_start();
+}