Add a public API for registering an error

[kddnewton]

Hey there - I was wondering if it might be possible to add a public API in order to register spell checkers. It's a pretty small change, but ergonomically it feels better than merging into a hash. I've added did_you_mean functionality to both thor and rubygems and it would feel better to call a blessed method.

Let me know what you think! Thanks for the great tool!

[yuki24]

Thanks for the PR! I'm very much in favor of adding a concrete interface for registering an error. I thought about the same thing when I was working on another gem called did_you_mean-activerecord that basiaclly does exactly this.

The only tiny thing I have in mind is whether or not the error class should always prepend Correctable. The correct_error method could check it so that it prepends only when necessary. This way the SPELL_CHECKERS constant could be made private and the NoMethodError could be registered the same way other are registered.

Overall this is great and thanks again for your PR!

[kddnewton]

Yeah that's a good idea. Just updated the PR.

[yuki24]

Awesome, thanks!