Fix nested commands in interactive mode triggering prompt2 (issue #19)

Save and restore cmd_mode across interpolation parsing so that
luaX_readcommandcont resumes in the correct mode after a nested
backtick command inside ${...}.

llex.h

@@ -83,6 +83,7 @@ typedef struct LexState {
   TString *brkn;  /* "break" name (used as a label) */
   TString *glbn;  /* "global" name (when not a reserved word) */
   lu_byte cmd_mode;  /* 0=normal, 1=backtick command, 2=interactive command */
+  lu_byte saved_cmd_mode;  /* cmd_mode to restore after interpolation */
 } LexState;