Has multiple ways of configuring the engine's components

[Sharktheone]

We want to have multiple options here. The current system where you create your own struct and implement the Module Config by hand is quite tedious. So there are some ideas

• let the engine have feature-flags that decide what to use on the default config. Then you can specify what components you are using in the Cargo.toml
• have a macro that can generate the config for you, but you can still override things

engine_config!(EngineConfiguration, html, taffy, cario, document);


impl HasRequestAgent for EngineConfiguration {
    type RequestAgent = CustomRequestAgent;
}