Files struct should allow for a name parameter for use with url_for

[brassy-endomorph]

I want to be able to create URLs for my static assets during requests, and this should be done here:

actix-web/actix-files/src/files.rs

Lines 339 to 343 in d8df60b

	let rdef = if config.is_root() {
	ResourceDef::root_prefix(&self.mount_path)
	} else {
	ResourceDef::prefix(&self.mount_path)
	};

This is somewhat bothersome. If I want routes, I have to do something cumbersome to work around this. If there's a a clear way to do this, it should be documented.