debugging: implement a NOP command which just spits out its argument

You can use this in testcases to mark specific sections:
  $i3->command('nop before trying to crash')->recv;
leads to the following output in the i3 logfile:
  -------------------------------------------------
    NOP: before trying to crash
  -------------------------------------------------

src/cmdparse.l

@@ -120,6 +120,7 @@ grow                            { return TOK_GROW; }
 px                              { return TOK_PX; }
 or                              { return TOK_OR; }
 ppt                             { return TOK_PPT; }
+nop                             { BEGIN(WANT_WS_STRING); return TOK_NOP; }
 restore                         { BEGIN(WANT_WS_STRING); return TOK_RESTORE; }
 mark                            { BEGIN(WANT_WS_STRING); return TOK_MARK; }