Introduce `RequestInterceptor` to replace `RequestHeaders`.

In addition to global headers (which were doable before), this allows for global
path param replacement in URLs.

This has cropped up in a few APIs over the internet and we're traditionally not provided
a good path to implementation. Before you would have to manually move the global param
around your application to every method call site onto a Retrofit interface. This new
interface provides a single place to manage these global params along with other
global headers (such as for authentication).

Given:

```java
@GET("/show/{apiKey}/{showId}/episodes")
List<Episode> episodes(@Path("showId") String showId)
```

Usage:

```java
class FooInterceptor implements RequestInterceptor {
  @Inject @ApiKey String apiKey;

  @Override public void intercept(RequestFacade request) {
    request.addPathParam("apiKey", apiKey);
  }
}
```

This also has the added benefit of saving an extra `List` allocation for global headers
since they are now set directly on the underlying `Request` via the facade interface.