Add context to ErrorHandler

[pebo]

In order to correlate custom ErrorHandler logging with requests if would be very helpful if the ErrorHandler signature was expanded to include context.Context e.g.

type ErrorHandler func(ctx context.Context, w http.ResponseWriter, message string, statusCode int)

This however is a breaking change so another option could be to expand Options with a ErrorHandlerWithContext field.

[pebo]

Added two example PRs for this.

Ran into the missing context when migration to chi from gin which has a context in the validation middleware:

type ErrorHandler func(c *gin.Context, message string, statusCode int)

[mikeschinkel]

There is definitely a need for more info in the error handler.

Context is one such need, but the current matched Route, the Request, and the Request's Headers — like Accept — are others.

I created a PR (#15) that (I think) addresses @pebo's use-case in this issue but also addresses others, and in a manner that will allow adding other info if such info is found to be needed by others in the future. Hopefully PR #15 can supersede PRs #13 and #14 since they change error handling but don't address the use-cases that #15 address.