[RFC][DO NOT MERGE] Add GuessByRegexp.

[hanabokuro]

This is an experimental implementation.

[syucream]

GuessByRegexp looks useful guess rule! I'd like to support that but we need to think how to read regexp pattern string.

We can some options, like below:

• cli options
  • It's a simple way, but giving complicated regexp's become hard.
  • I'm imaging to give a regexp-specific option name, like -regexp-pattern=... to indicate it's relating to regexp rule explicitly.
• config file
  • It helps us to write complicated regexp pattern.
  • But hakagi need handling to read the files. (It's a little problem?)
• environment variable
  • It's similar to cli options.

I recommend we try the config file way because it looks flexible and an additional cost is only reading files. What do you think? Is it acceptable? 🤔

[hanabokuro]

I think that cost is acceptable.
I will try to write a code that handle config.