Support optional regex using RegexBuilder

`regex` crate has `RegexBuilder` to configure behavior of `Regex`.

I may need options for users who want to use this features.

## Pre-image

```
[[files]]
path = "example.txt"
regex = true
regex_options = { crlf = true }
...
```

## Refs

- #8
- https://docs.rs/regex/latest/regex/struct.RegexBuilder.html