[todo] Add Middleware/Plugin Support

[headzoo]

It would be nice if Surf had a built in mechanism for middleware/plugins. It would be nice if developers could set plugin modules that perform common tasks and/or add functionality. For example:

• OAuth support
• Logging into popular sites (Github, Reddit, GMail, etc)

Possible syntax:

// GithubLogin implements a Surf plugin for Github authentication.
// When requests are made to the github.com domain, the plugin
// performs user authentication automatically if the user isn't
// logged in already.
type GithubLogin struct {
    username string
    password string
}

... implement methods

func main() {
    bow := NewBrowser()

    login := middleware.NewGithubLogin("username", "password")
    bow.AddMiddleWare(login)

    err := bow.Open("https://github.com")
    if err != nil {
        panic(err)
    }
}

[lxt2]

Thoughts on using something like https://github.com/f2prateek/train? If #39 gets merged this would already be possible :)