Topic/accesslog private methods

I am creating Plack::Middleware::AccessLog::Custom which allows the customization that I did in pull request #381 that you rejected.  Here in this branch I have pulled out the handler methods so they can be called by the subclass.  There is no change in functionality here, other than adding support for uppercased block handlers.

I hope this is acceptable, as it will make it possible to log some things that are currently not loggable, such as additional fields from $env (such as CONTENT_LENGTH, CONTENT_TYPE).