Support optional regex using RegexBuilder

[attakei]

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

• Support regex replacement #8
• https://docs.rs/regex/latest/regex/struct.RegexBuilder.html