Support skipping certain path(s) with config skip_paths

[choznerol]

Thanks for creating this gem!

This PR support skipping certain paths with a new configuration option skip_paths.

skip_paths supports both String and Regexp, and will be matched against env['PATH_INFO'].

Example usage:

Rails.application.config.json_key_transformer_middleware.skip_paths = [/^\/admin/, '/graphql']

# Will not transform neither `GET /admin/foo?bar_id=1` nor `POST /graphql`

Some thoughts:

1. Perhaps skip_paths is a better name? I updated excluded_paths to skip_paths
2. I also considered accepting a proc to achieve the same goal (e.g. should_skip = ->(env) { env['PATH_INFO'].match?('/foobar') }) but thinking this could be a bit confusing, and users may not need such flexibility. If providing another advanced config should_skip sounds good I can send another PR for that.