NewFeedWatcher returns a new FeedWatcher instance.

After allows for stubbing out in test.

ErrAlreadyCrawlingFeed is when you are already Crawling a feed.

ErrCrawlerNotAvailable is when the crawler channel would block.

ErrMailDeliveryFailed captures the specific reason for a mail failure.

FeedCrawlRequest is how to request a URL to be crawled by a crawler instance.

FeedCrawlResponse is we get back from the crawler.

FeedWatcher controlls the crawling of a Feed.