Add DownloaderOptions to allow customization of a download when a request results in a download

Implement `DownloaderOptions` though request flow to allow optional customization of request properties on every request.

This consolidates the `loadFromLocalCacheOnly` argument from the `Downloader` interface into `DownloaderOptions`.
* Note: This breaks backwards compatibility of the `Downloader` interface. If this is unacceptable, we can discuss other ideas for passing options to the downloaders.

This is offered as an alternate solution to initializing Picasso with a custom Downloader when it is necessary to customize every request differently. Issues:
* https://github.com/square/picasso/issues/459
* https://github.com/square/picasso/issues/693
* https://github.com/square/picasso/issues/84