fix invalid option message, add option to README, ignore tmenu

The error message (second output line) for unknown options are redundant and incorrect.

```
$ ./tmenu -x
./tmenu: invalid option -- 'x'
./tmenu: unknown option '?'
```

It should be `optopt` in line 235, but since `getopt` already prints out error message (first output line), so I just removed it.