Add `-o` and `--reorder` option to reorder keys with `--keys` option

It reorders keys with `--keys` option like:

```plain
$ echo "key1:val1\tkey2:val2" | ltsview -l -k key2,key1 --reorder
key2:val2	key1:val1
```

I assigned `-o` to the short option because `-r` is used for `--regexp` option, but it somehow doesn't look good to me. If you have alternatives, please tell me them.