Add a --arg option to allow variables to be passed from the cmdline.

Closes #107

compile.h

@@ -40,6 +40,8 @@ block gen_condbranch(block iftrue, block iffalse);
 block gen_and(block a, block b);
 block gen_or(block a, block b);
 
+block gen_var_binding(block var, const char* name, block body);
+
 block gen_cond(block cond, block iftrue, block iffalse);
 
 block gen_cbinding(const struct cfunction* functions, int nfunctions, block b);