Remove custom formatter

[ioquatix]

https://github.com/ruby/error_highlight/blob/9fbaa8ab7c09257320afb39ae4619251362291b1/lib/error_highlight/formatter.rb#L16

I believe it would be prudent at least for the first stable release of this gem, to remove the ErrorHighlight#formatter override.

I believe it will be very hard to use in practice and adds unnecessary public interface and complexity.

[eregon]

Agreed, ErrorHighlight is an internal gem, it should not expose any public method. Let's design this kind of things properly and not make them ErrorHighlight-specific.