add RequestInterceptor

When you need to change all requests, regardless of their target, you'll want to configure a `RequestInterceptor`.
For example, if you are acting as an intermediary, you might want to propagate the `X-Forwarded-For` header.

```
@Module(library = true)
static class ForwardedForInterceptor implements RequestInterceptor {
  @Provides(type = SET) RequestInterceptor provideThis() {
    return this;
  }

  @Override public void apply(RequestTemplate template) {
    template.header("X-Forwarded-For", "origin.host.com");
  }
}
...
GitHub github = Feign.create(GitHub.class, "https://api.github.com", new GsonModule(), new ForwardedForInterceptor());
```