Allow buffalo to accept preconfigured http.Server(s)

[bjwschaap]

In order to be able to set custom properties for a http.Server (e.g. TLS options, Read-, Write- and IdleTimeout), it would be nice for the buffalo App to accept one or more preconfigured http.Server instances, instead of creating a default one.

e.g.:

srv := &http.Server{
    ReadTimeout:  5 * time.Second,
    WriteTimeout: 10 * time.Second,
    IdleTimeout:  120 * time.Second,
    TLSConfig:    &tls.Config{
        PreferServerCipherSuites: true,
        CurvePreferences: []tls.CurveID{
            tls.CurveP256,
            tls.X25519, // Go 1.8 only
        },
    },
}

app := actions.App()
if err := app.Serve(srv); err != nil {
    log.Fatal(err)
}