add WrapProcess to allow users to alter command processing

[jrmarkle]

This could be used to update metrics or debug/logging (#259).

There are many ways this could be done. I chose a way that involves minimal changes.

Example usage:

    client = redis.NewClient(...)

    client.WrapProcess(func(oldProcess func(cmd redis.Cmder) error) func(cmd redis.Cmder) error {
        return func(cmd redis.Cmder) error {
            before := time.Now()

            err := oldProcess(cmd)

            if err != nil {
                log.Println(`error running redis command "`, cmd, `": `, err)
            } else {
                log.Println(`running redis command "`, cmd, `" took `, time.Since(before))
            }

            return err
        }
    })

[vmihailenco]

I like this - thanks!

[vmihailenco]

Released in v4.1.3