Add --nth and --delimiter option

### Summary

`--nth` and `--delimiter` option can be used to limit the search scope to the N-th token of each item.

```sh
# ctags search
grep -v '^!' tags | cut -f1-2 | fzf -n1 | cut -f2
```

### Tokenization

When `--delimiter` option is not given, tokenization follows the way awk does it by default.
- Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored.

http://www.gnu.org/software/gawk/manual/html_node/Field-Splitting-Summary.html#Field-Splitting-Summary

### Negative indexing

`--nth` can take a negative integer. In this case search scope is limited to be the "N-th to the last" token.

```sh
# Match only in the tail of the file name
fzf -n-1 -d/
```